Altium import, nets, canonical pins, component kinds, set/$var, scrollback, source modal.

Major additions, all wired end-to-end with doctest coverage:

- Altium netlist importer (`imports/import_altium.{hpp,cpp}`): two-pass
  parser for `[ ]` parts and `( )` signals; `System::Load` no longer has
  the IMPORT_ALTIUM hole.
- `duplicate <src> <dst>` deep-copies a module (signals, parts, pins,
  rewired signals); connections excluded by design.
- Nets (`system/nets.{hpp,cpp}`): BFS over `Connection::pin_map` to
  return the transitive (Module, Signal) closure. `verify` extended with
  a second pass flagging Power↔GndShield inconsistencies in bridged
  nets; new `net <module> <signal>` command for inspection.
- Canonical pin names (`system/pin_name.{hpp,cpp}`): zero-padded digit
  suffix lets A1 ↔ A001 pair via `IdentityTransform` and
  `CheckIdentityCompatible` without losing the imported notation.
- Component classification (`system/component_kind.{hpp,cpp}`):
  `Part::kind` inferred at construction from the reference-designator
  prefix (longest-match: LED/TP/SW/FB/MK/MP/MH/HS/RA/RN/RP/RV first,
  then R/C/L/F/D/Q/U/J/P/Y/X/S).
- Identity wiring tolerance: `CheckIdentityCompatible` accepts the
  subset case (typical when one importer drops NC pins, e.g. Altium)
  and surfaces orphans as an info string. `FillIdentityNCs`
  materialises orphan canonical positions as NC pins on the missing
  side at connect time.
- Connector layout preparation: `pin_layout(kind)` and
  `FillPartFromLayout(part, kind)` stubs in `pin_role`, called from
  `set-type`. Empty today; populate alongside `vpx_3u_role`.
- TUI scrollback: PageUp/PageDown step 10 lines, Home/End jump to
  ends; `Print()` snaps back to the tail.
- `set <name> <value>` declares session variables; `$name` / `${name}`
  expanded inside `Finalize` between canonical-form recording and the
  action call — history and script-save preserve `$var` references.
- Long `source` scripts now show a centred "Computing…" modal with a
  N/M progress counter. Driven by a ticker thread that posts one
  paced `Event::Special` per processed line, ack'd by the main thread,
  so heavy lines don't backlog ticks and freeze the counter.

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

src/tui/tui.hpp

@@ -1,6 +1,7 @@
 #ifndef _TUI_HPP_
 #define _TUI_HPP_
 
+#include <atomic>
 #include <deque>
 #include <functional>
 #include <map>
@@ -41,12 +42,14 @@ class Tui {
     std::string input;
     int cursor_pos;
     int history_idx;
+    int scroll_offset;  ///< Lines scrolled up from the tail; 0 = follow newest output.
     bool quit;
     bool in_source;
 
     std::unique_ptr<System> sys;
     std::deque<Prompt> pending;
     std::map<std::string, CommandSpec> commands;
+    std::map<std::string, std::string> vars;  ///< $var-style substitution table.
 
     // ---- Screen orchestration ----
     int screen_idx;
@@ -85,6 +88,17 @@ class Tui {
     std::string explore_header;
     int explore_focus_idx;
 
+    // ---- Source-file loading (event-driven, one line per tick) ----
+    std::atomic<bool> loading;         ///< true while a script is being processed; read by tick thread.
+    std::atomic<bool> tick_in_flight;  ///< main thread acks each tick by clearing this; ticker waits.
+    std::string loading_filename;
+    std::vector<std::string> loading_lines;
+    size_t loading_idx;
+    int loading_executed;
+    int loading_lineno;
+    bool loading_prev_in_source;
+    ftxui::ScreenInteractive *screen_ptr;  ///< set in Run() so Source() can post events.
+
     // ---- Set-type screen state ----
     std::vector<std::string> settype_modules;
     int settype_m_idx;
@@ -117,6 +131,8 @@ private:
     void LoadHistory();
     void AppendHistory(const std::string &cmd);
     void Source(const std::string &filename);
+    void ProcessNextSourceLine();
+    std::string ExpandVars(const std::string &s) const;
 
     // Completion (completion.cpp)
     void CompleteCommand(size_t start = 0);