Signal types, pin role expectations, and a doctest suite.

Domain
- Signal carries a SignalType (Power/GndShield/Other), auto-inferred
  from the name in Signal::Signal via infer_signal_type. Override with
  the new `set-signal-type` command.
- SignalType extracted to its own header so Pin can store an
  `expected_signal_type` without a pins↔signals include cycle.
- pin_role(connector_type, pin_name) → SignalType lookup, called from
  set-type to populate each Pin's expected_signal_type. The VPX 3U
  table is currently a stub (returns Other).
- New `verify` command walks typed parts and reports pins whose
  connected signal's type doesn't match the expectation.
- ODS importer no longer drops pins with empty signal column — they
  stay in the part as NC, matching the rule "a pin is either NC or
  connected to a signal".
- persist: new S tag for non-default signal type overrides.

Tests
- doctest v2.4.11 via FetchContent (with CMAKE_POLICY_VERSION_MINIMUM
  shim, doctest's CMakeLists has a too-old floor for current CMake).
- Source files moved into a static library `essim_lib` so both `essim`
  and `essim_tests` reuse the same compilation. main.cpp is the only
  file kept out of the lib.
- Layer 1 (pure helpers): ToLower, LongestCommonPrefix, Tokenize,
  NaturalLess (numeric/case/leading-zero edge cases + total-order
  invariants), signal_type round-trips and infer_signal_type families,
  VpxTransform registry + symmetry + reference-table mapping for
  connector P0 row 1, IdentityTransform same-name wiring.
- Layer 2 (round-trip): build a synthetic 2-module system in code,
  save → restore → assert modules / parts / connector_types / NC pins
  / signal type overrides / connections + pin_map are all preserved.
- Tui::Tokenize moved to a free function in tui_helpers so tests can
  call it without dragging ftxui into the unit-test layer.
- 27 test cases, 123 assertions, ~150 ms.

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

CLAUDE.md

@@ -81,7 +81,15 @@ Built-in commands: `new`, `load`, `save`, `restore`, `source`, `script-save`, `c
 
 Pending prompts (from incomplete inline commands) are NOT considered interactive and are filled by subsequent script lines, the way you'd expect. Lines starting with `#` and blank lines are skipped; leading/trailing whitespace is trimmed; `~/` is expanded.
 
-`save` / `restore` (`src/system/persist.{hpp,cpp}`): tab-delimited line format, no extra deps. Tags: `M` (module), `P` (part with `connector_type`), `N` (pin → signal name; empty = NC), `C` (connection header with endpoints + `transform_name`), `W` (wire pair within the current connection). `restore` replaces `Tui::sys` entirely (`unique_ptr::reset`). Names are stored as-is — must not contain TAB or newline (true for the EE netlists we ingest). Format is versioned by the `# essim system snapshot v1` header for future compatibility.
+`save` / `restore` (`src/system/persist.{hpp,cpp}`): tab-delimited line format, no extra deps. Tags: `M` (module), `P` (part with `connector_type`), `N` (pin → signal name; empty = NC), `S` (signal → type override; only emitted for non-default), `C` (connection header with endpoints + `transform_name`), `W` (wire pair within the current connection).
+
+**Signals** carry a `type` (`SignalType::Power | GndShield | Other`) auto-inferred from the name in `Signal::Signal` via `infer_signal_type` (heuristic: GND/GROUND/SHIELD/CHASSIS → GndShield; PWR/VCC/VDD/VEE/VSS/VBAT/VS_/VS3_*/+/- prefixes → Power; else Other). Override with `set-signal-type <module> <signal> <power|gnd|other>`. The explore screen shows the type in the signal detail header.
+
+**Pin role expectations**: every Pin carries an `expected_signal_type` populated by `set-type` from a per-(connector_type, pin_name) lookup (`src/system/pin_role.{hpp,cpp}`). The framework is wired end-to-end; the actual VPX 3U lookup table is currently a stub returning Other for all positions — fill in `vpx_3u_role(col, row, idx)` with the real VITA 46 layout when needed. The `verify` command walks all typed parts and reports pins whose connected signal's type doesn't match the expectation.
+
+`SignalType` lives in its own header `src/system/signal_type.hpp` (extracted from signals to avoid a pins↔signals include cycle).
+
+**Pins** are either NC (`signal() == nullptr`) or connected to exactly one signal. The ODS importer creates a Pin for every row that has a non-empty pin name, even when the signal column is empty or `"NC"` — the pin stays in the Part as NC. `restore` replaces `Tui::sys` entirely (`unique_ptr::reset`). Names are stored as-is — must not contain TAB or newline (true for the EE netlists we ingest). Format is versioned by the `# essim system snapshot v1` header for future compatibility.
 
 **Connector types & transforms**: every `Part` carries a `connector_type` string (default `""`, set via the `set-type` command — inline `set-type m p kind` or bare which opens a TUI screen with module menu, part filter+menu, type input, list of types already in use, and an Apply button). When `connect` validates a pair, it consults `TransformRegistry::lookup(p1->connector_type, p2->connector_type)` (defined in `src/system/transform.{hpp,cpp}`) — both directions of the pair are tried. If neither is registered, an `IdentityTransform` fallback wires each pin of A to the same-name pin of B (when present). The resulting `(Pin*, Pin*)` list and the transform's name are stored on the `Connection` (`pin_map`, `transform_name`). To register a real transform: define a `Transform` subclass in `transform.cpp` and call `TransformRegistry::get().add("kindA", "kindB", new MyTransform())` at init — there's no startup hook for this yet, so a small `RegisterBuiltinTransforms()` helper is the natural place to add when more types appear.