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>

CMakeLists.txt

@@ -26,17 +26,40 @@ FetchContent_MakeAvailable(ftxui)
 find_package(libzip REQUIRED)
 find_package(pugixml REQUIRED)
 
-file(GLOB_RECURSE ALL_SOURCES "src/*.cpp")
+# Library target = everything except main.cpp; reused by `essim` and `essim_tests`.
+file(GLOB_RECURSE LIB_SOURCES "src/*.cpp")
+list(REMOVE_ITEM LIB_SOURCES "${CMAKE_CURRENT_SOURCE_DIR}/src/main.cpp")
 
-add_executable(essim ${ALL_SOURCES})
-
-target_include_directories(essim PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/src)
-
-target_link_libraries(essim
-  PRIVATE
+add_library(essim_lib STATIC ${LIB_SOURCES})
+target_include_directories(essim_lib PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/src)
+target_link_libraries(essim_lib
+  PUBLIC
     ftxui::screen
     ftxui::dom
     ftxui::component
     libzip::zip
     pugixml::pugixml
 )
+
+add_executable(essim src/main.cpp)
+target_link_libraries(essim PRIVATE essim_lib)
+
+# Tests
+include(CTest)
+if(BUILD_TESTING)
+  set(CMAKE_POLICY_VERSION_MINIMUM 3.5)
+  FetchContent_Declare(doctest
+    GIT_REPOSITORY https://github.com/doctest/doctest.git
+    GIT_TAG v2.4.11
+    GIT_SHALLOW TRUE
+  )
+  FetchContent_MakeAvailable(doctest)
+  unset(CMAKE_POLICY_VERSION_MINIMUM)
+
+  file(GLOB TEST_SOURCES "tests/*.cpp")
+  if(TEST_SOURCES)
+    add_executable(essim_tests ${TEST_SOURCES})
+    target_link_libraries(essim_tests PRIVATE essim_lib doctest::doctest)
+    add_test(NAME essim_tests COMMAND essim_tests)
+  endif()
+endif()