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>

src/tui/shell.cpp

@@ -41,22 +41,6 @@ void Tui::CancelPending() {
     Print("(cancelled)");
 }
 
-std::vector<std::string> Tui::Tokenize(const std::string &s) {
-    std::vector<std::string> out;
-    std::string cur;
-    bool in_q = false;
-    for (char c : s) {
-        if (c == '"') { in_q = !in_q; continue; }
-        if (!in_q && std::isspace((unsigned char)c)) {
-            if (!cur.empty()) { out.push_back(std::move(cur)); cur.clear(); }
-        } else {
-            cur.push_back(c);
-        }
-    }
-    if (!cur.empty()) out.push_back(std::move(cur));
-    return out;
-}
-
 void Tui::Submit() {
     if (!pending.empty()) {
         if (input.empty()) { Print("(empty — Esc to cancel)"); return; }