Export command (CSV + ODS), file dialog, error modal, path persistence.

New user-facing features:
- `export connections <file>` writes a tabular dump of every wire pair:
  connection, transform, left/right module/part/pin/signal/type/suspect,
  mixed-types flag. Dispatch on extension: `.csv` (flat file) or `.ods`
  (one sheet per connection). Any other extension shows an error and
  writes nothing.
- Bare `export` (or dashboard `[x]`, or palette `export`) opens an
  interactive file-picker dialog with a CSV/ODS toggle at the top.
  Picking a filter rewrites the filename's extension. Last-used
  directory and filename are remembered per-call-site.
- Two new CLI flags on the binary: `--source FILE` to run a script at
  boot, `--restore FILE` to restore a snapshot at boot. Combinable.

Reusable infrastructure:
- `OdsWriter` (`src/imports/ods_writer.{hpp,cpp}`): minimal .ods writer
  using libzip + pugixml (already in the build for the importer).
  Multi-sheet workbook of string cells. ~180 lines, no new dep.
- Generic file-picker dialog (`screen_filedialog.cpp`): one Modal
  reused for any "pick a path" interaction via
  `OpenFileDialog(title, persist_key, default_filename, filters, cb)`.
  Validates the picked extension against the filter whitelist;
  unknown ones stay in the dialog with a status message. Persists
  (dir, filename) per `persist_key`.
- Generic error modal (`screen_error.cpp`, `ShowError(msg)`): centred
  red-titled popup, dismissable with Esc/Enter. Used by the export
  failures (open-for-write, ODS save, unknown extension/kind);
  ready for adoption elsewhere.
- Per-key path persistence (`SaveLastUsed`/`LoadLastUsed` in
  `shell.cpp`): two-line file per key under the user-data dir.
- `UserDataDir()` extracted from the history path helper so the new
  per-key persistence shares the same XDG/AppData logic.
- New help-screen topic "Export"; user-facing `doc/user/analysis.md`
  gains an "Exporting" section; `DESIGN.md` gains a generics
  section covering the dialog / error modal / persistence / ODS
  writer; `DumpCommandsMd` now respects the `hidden` flag (the
  `connect` alias no longer appears in the auto-gen reference).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

DESIGN.md

@@ -38,11 +38,12 @@ src/
     analysis.{hpp,cpp}     analyze_system → AnalysisReport (diff pairs, buses, anomalies)
     persist.{hpp,cpp}      save / restore (tab-delimited)
     system.{hpp,cpp}       System: owns Modules + Connections, exposes Load()
-  imports/               -- adapters that populate the domain
+  imports/               -- adapters that populate or emit the domain
     import_base.hpp        ImportBase interface
     import_mentor.{hpp,cpp} Mentor Graphics netlist parser
     import_altium.{hpp,cpp} Altium netlist parser (`[ ]` parts, `( )` signals)
     import_ods.{hpp,cpp}   ODS spreadsheet pinout (libzip + pugixml)
+    ods_writer.{hpp,cpp}   Minimal .ods writer (multi-sheet, string cells)
   tui/                   -- FTXUI shell, split by responsibility
     tui.{hpp,cpp}          Class Tui (state + Run() orchestrator + screen-mode event dispatcher)
     tui_helpers.{hpp,cpp}  Free helpers: ToLower, NaturalLess, LongestCommonPrefix
@@ -56,6 +57,9 @@ src/
     screen_dashboard.cpp   BuildDashboardScreen (read-only system overview)
     screen_analyze.cpp     BuildAnalyzeScreen (anomalies / groups / power decisions)
     screen_palette.cpp     BuildPaletteModal (global Ctrl-P fuzzy launcher)
+    screen_filedialog.cpp  BuildFileDialog + OpenFileDialog (reusable file picker)
+    screen_error.cpp       BuildErrorModal + ShowError (centred error popup)
+    screen_help.cpp        BuildHelpScreen (topic-driven feature reference)
     screen_sigtype_modal.cpp BuildSignalTypeModal (popup attached to explore via Modal())
 doc/classes.puml         -- PlantUML class diagram
 ```
@@ -158,6 +162,26 @@ Exposed as the `analyze` shell command which prints groups (sorted by module + l
 
 Everything is recomputed every frame so manual overrides via the signal-type popup are reflected immediately. Esc returns to the dashboard. The dashboard's previous `[v]erify` letter shortcut was removed — its content is fully covered by this screen. The textual `verify` / `analyze` commands still exist for scripts.
 
+**Generic file-picker dialog** (`screen_filedialog.cpp`): one reusable modal for every "pick a path" interaction. State lives in `Tui::file_dialog` (a single `FileDialogState`); attached to the tab tree via `Modal(BuildFileDialog(), &file_dialog.open)` in `Run()`. API:
+
+- `OpenFileDialog(title, persist_key, default_filename, filters, on_confirm)` — opens the modal, restoring the last-used `(dir, filename)` for `persist_key` if previously saved.
+- `ConfirmFileDialog()` runs on Enter on the OK button: validates against the filter whitelist (rejects unknown extensions with an in-dialog status message), persists `(dir, filename)` under `persist_key`, closes the modal, then calls `on_confirm(full_path)`.
+
+The optional `filters` vector (`{label, extension}` pairs) renders a horizontal Toggle at the top of the dialog. The first frame after Open seeds the filter index from the filename's current extension; subsequent index changes rewrite the filename extension so the caller's extension-based dispatch picks the right format. Empty filter list ⇒ no Toggle shown, no extension validation.
+
+Today the only caller is `export` (`{"CSV", ".csv"}, {"ODS", ".ods"}` filters, key `export.connections`), but future callers (`save`, `restore`, `source`, …) plug in by changing four arguments.
+
+**Generic error modal** (`screen_error.cpp`, `Tui::ShowError(msg)`): centred `borderRounded` popup with red title, the message (wrapped via `paragraph`), and an OK button. Esc / Enter dismiss. Stacked at the top of the Modal chain in `Run()` so it overlays every screen and every other modal. The error is also `Print()`-ed to the console log, so it remains inspectable after dismissal. Used by the export action (unknown extension, open-for-write failure, ODS save failure, unknown kind); other actions can adopt by replacing user-visible `Print("...failed...")` calls with `ShowError(...)`.
+
+**Per-key path persistence** (`SaveLastUsed(key, dir, filename)` / `LoadLastUsed(key, &dir, &filename)` in `shell.cpp`): each key writes a tiny two-line file (`dir\nfilename\n`) under `UserDataDir() / <key>.last`. `UserDataDir()` is the cross-platform `XDG_DATA_HOME` / `LOCALAPPDATA` etc. helper also used by the command history file. Free functions, not Tui members, so any module (the file dialog today; could be the script-save buffer or the save command tomorrow) can use them with the same minimal API.
+
+**ODS writer** (`src/imports/ods_writer.{hpp,cpp}`): minimal OpenDocument Spreadsheet writer. Backed by libzip + pugixml (both already pulled in by the ODS importer). Class hierarchy is two structs:
+
+- `OdsSheet` — sparse row-major grid of string cells (`set(row, col, value)`).
+- `OdsWriter` — owns the sheets, emits a valid `.ods` archive with `mimetype` (stored uncompressed, magic header), `META-INF/manifest.xml`, and `content.xml`.
+
+`content.xml` is built with pugixml: one `<table:table>` per sheet, each row a `<table:table-row>` of string `<table:table-cell>` cells. String-only by design (no numbers, dates, formulas, styles, merges) — the format minimum for "rectangular text data" is concise enough to live in ~200 lines without depending on a heavyweight ODS library.
+
 **Command palette** (`screen_palette.cpp`): a global modal launcher attached to the whole tab tree via `tab | Modal(BuildPaletteModal(), &palette_open)` in `Run()`. Trigger: `Event::CtrlP` (FTXUI Input does not consume Ctrl-P, so the outer `CatchEvent` reliably picks it up first). Behaviour: a single Input bound to `palette_query` plus a result list rebuilt on every frame. Indexes three kinds of entries: commands (from the `commands` map), modules and per-module signals (qualified as `module/signal`). Fuzzy match is subsequence-based, case-insensitive: lower score wins, computed as `first_match_position * 100 + sum_of_gaps`. Kinds are biased by a constant offset (commands +0, modules +1000, signals +2000) so command matches come first when scores tie. Output capped at 20 rows to keep render cheap on big systems. Activation (`Enter`): commands → `Dispatch(name)` (which dispatches like the shell, including opening interactive screens), module → prefill `explore_*` state and jump to `screen_idx = 4`, signal → prefill `net_modules` + seed `net_sig_filter` to the exact signal name and jump to `screen_idx = 5`. `Esc` closes the palette. While the palette is open, the outer `CatchEvent` cedes events to it so Tab/Esc/etc. don't leak into the underlying screen.
 
 **Dashboard** (`screen_dashboard.cpp`, `dashboard` command, `screen_idx = 4`): read-only system overview. Single Renderer, no Input child. Recomputes everything per frame (cheap on realistic sizes): counters (modules/parts/signals/connections), three health rows (verify pin-role mismatches, bridged-net inconsistencies, NC orphans — green check / yellow warning prefix), an analysis summary line (diff pairs / buses / anomaly count, coloured if non-zero), and a per-module table (parts / signals / `connector_type`-tagged parts). Letter shortcuts handled in the outer `CatchEvent`: `c`=console, `p`=plug (connect), `t`=set-connector-type, `e`=explore, `a`=analyze, `q`=quit. `Esc` is swallowed on the dashboard (home). The dashboard is `interactive = true`, `scriptable = false`; running `dashboard` inside `source` aborts the script.