build: split core/ from frontends/; prepare for multiple GUI/TUI targets

Reorganise the tree into business vs frontend as separate directories:
  src/core/{domain,imports,app}   (was system/, imports/, app/)
  src/frontends/tui/              (was tui/ + main.cpp)
  tests/tui/                      (the FTXUI-coupled helper test)
All cross-dir #include paths rewritten; same-dir includes untouched.

CMake: essim_core is the frontend-agnostic business library — links libzip,
pugixml and bsdl, NO GUI toolkit. Each frontend is a self-contained
src/frontends/<name>/ (own CMakeLists, toolkit, main.cpp) that links
essim_core, selected with -DESSIM_FRONTEND=<name> (default tui; 'none' = core +
tests only, no toolkit fetched). FTXUI moved into the tui frontend. Tests are
split: essim_tests links essim_core (no FTXUI), essim_tui_tests links essim_tui.

Verified: default tui build green (ctest 2/2); ESSIM_FRONTEND=none builds the
core + tests with FTXUI never fetched and no `essim` binary.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/core/domain/pin_name.hpp

@@ -0,0 +1,25 @@
+#ifndef _PIN_NAME_HPP_
+#define _PIN_NAME_HPP_
+
+#include <string>
+
+// Canonical form of a pin name for cross-card matching: <leading non-digits>
+// + <pure-digit suffix zero-padded to width 3>. Returns the original name
+// unchanged when no digit suffix is present, or when the suffix mixes digits
+// with other characters.
+//
+// Examples:
+//   "A1"    -> "A001"
+//   "A001"  -> "A001"
+//   "AB12"  -> "AB012"
+//   "12"    -> "012"
+//   "VCC"   -> "VCC"
+//   "A1B"   -> "A1B"     (mixed suffix, not canonicalised)
+//   "P3.3V" -> "P3.3V"   (mixed suffix)
+//
+// Used by IdentityTransform and CheckIdentityCompatible to match pin names
+// that differ only in zero-padding. Pin::name itself is preserved as-imported
+// so the original schematic notation survives in the UI and on disk.
+std::string canonical_pin_name(const std::string &name);
+
+#endif // _PIN_NAME_HPP_