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_role.cpp

@@ -0,0 +1,92 @@
+#include "pin_role.hpp"
+
+#include "parts.hpp"
+#include "pin_name.hpp"
+#include "pins.hpp"
+
+#include <cctype>
+#include <exception>
+#include <string>
+#include <unordered_set>
+#include <vector>
+
+// VPX 3U built-in pin role tables.
+//
+// NOTE: real VITA 46 pin roles are connector-/profile-specific (data lanes,
+// power planes, GND chassis, etc.). The placeholders below are intentionally
+// minimal — fill in the actual per-(col,row) roles for your design when the
+// reference is available; the rest of the chain (set-connector-type → verify) is
+// already wired through this single function.
+
+namespace {
+
+// Quick char column dispatch — returns Other when the column isn't recognised.
+SignalType vpx_3u_role(char /*col*/, int /*row*/, int /*connector_idx*/) {
+    // TODO: encode the real layout. Right now everything is "other"; verify()
+    // therefore reports nothing for VPX until this table is filled in.
+    return SignalType::Other;
+}
+
+bool parse_pin(const std::string &s, char &col, int &row) {
+    if (s.size() < 2) return false;
+    if (!std::isalpha((unsigned char)s[0])) return false;
+    for (size_t i = 1; i < s.size(); ++i)
+        if (!std::isdigit((unsigned char)s[i])) return false;
+    col = (char)std::toupper((unsigned char)s[0]);
+    try { row = std::stoi(s.substr(1)); }
+    catch (const std::exception &) { return false; }
+    return true;
+}
+
+}  // namespace
+
+PinSpec pin_role(const std::string &kind, const std::string &pin_name)
+{
+    PinSpec spec;
+    if (kind.empty()) return spec;
+
+    char col; int row;
+    if (!parse_pin(pin_name, col, row)) return spec;
+
+    SignalType st = SignalType::Other;
+    int idx = -1;
+    bool is_vpx_3u = false;
+    if (kind.rfind("vpx-3u-bkp-p", 0) == 0
+        || kind.rfind("vpx-3u-payload-p", 0) == 0) {
+        is_vpx_3u = true;
+        const std::string suffix = kind.substr(kind.size() - 1);
+        try { idx = std::stoi(suffix); } catch (const std::exception &) {}
+    }
+    if (is_vpx_3u && idx >= 0) st = vpx_3u_role(col, row, idx);
+
+    spec.function = function_from_signal_type(st);
+    spec.source   = SpecSource::ConnectorModel;
+    return spec;
+}
+
+std::vector<std::string> pin_layout(const std::string &kind)
+{
+    // TODO: enumerate the canonical pin set for known connector types,
+    // alongside `vpx_3u_role`. Empty for now — `FillPartFromLayout` becomes
+    // a no-op and the rest of the pipeline (verify, explore, identity wiring)
+    // works on whatever pins were imported.
+    (void)kind;
+    return {};
+}
+
+int FillPartFromLayout(Part *p, const std::string &kind)
+{
+    if (!p) return 0;
+    auto layout = pin_layout(kind);
+    if (layout.empty()) return 0;
+    std::unordered_set<std::string> existing;
+    for (auto &kv : *p) existing.insert(canonical_pin_name(kv.first));
+    int added = 0;
+    for (const auto &name : layout) {
+        if (existing.count(canonical_pin_name(name))) continue;
+        if (p->exists(name)) continue;
+        p->add(new Pin(name));
+        ++added;
+    }
+    return added;
+}