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/bsdl_model.hpp

@@ -0,0 +1,73 @@
+#ifndef _BSDL_MODEL_HPP_
+#define _BSDL_MODEL_HPP_
+
+#include "pin_model.hpp"
+#include "pin_spec.hpp"
+
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+class Part;
+struct bsdl;  // libbsdl C model — opaque here.
+
+// A parsed BSDL device reduced to essim's pin vocabulary. Value type: the
+// underlying libbsdl C model is consumed and freed during construction, so a
+// BsdlModel carries no C resources and copies/moves freely.
+class BsdlModel
+{
+public:
+    struct Port {
+        std::string  name;                              ///< Logical BSDL port name.
+        PinDirection direction = PinDirection::Unknown;
+        PinFunction  function  = PinFunction::Unknown;  ///< TAP role / power / ground / signal.
+        std::string  pad;                               ///< Physical package pin (PIN_MAP); "" if unmapped.
+    };
+
+    static BsdlModel from_file(const std::string &path);
+    static BsdlModel from_buffer(const std::string &text, const std::string &name = "bsdl");
+
+    bool valid() const { return ok_; }
+    const std::string &entity() const { return entity_; }
+    const std::string &error() const { return error_; }
+    const std::vector<Port> &ports() const { return ports_; }
+
+private:
+    static BsdlModel from_handle(struct bsdl *d);  // consumes (frees) d.
+
+    bool ok_ = false;
+    std::string entity_;
+    std::string error_;
+    std::vector<Port> ports_;
+};
+
+// Adapts a parsed BSDL device to the PinModel interface, indexing its ports by
+// both logical name and physical pad (so a netlist that names IC pins either by
+// signal or by ball both resolve). `layout()` is empty — BSDL drives specs, not
+// pin materialisation, since the netlist's pin naming may differ from the port
+// names.
+class BsdlPinModel : public PinModel {
+public:
+    explicit BsdlPinModel(const BsdlModel &model);
+    PinSpec spec_for(const std::string &pin_name) const override;
+    std::vector<std::string> layout() const override { return {}; }
+    SpecSource source() const override { return SpecSource::Bsdl; }
+
+private:
+    std::unordered_map<std::string, PinSpec> by_name_;
+    std::unordered_map<std::string, PinSpec> by_pad_;
+};
+
+// Outcome of binding a model onto a Part's pins.
+struct BsdlApplyReport {
+    int pins_total = 0;
+    int bound = 0;     ///< Ports matched to a pin (by name, then by physical pad).
+    int unbound = 0;   ///< Ports with no matching pin on the part.
+};
+
+// Set each matched Pin's `spec` (function / direction / pad, source = Bsdl) from
+// the model. A port is matched to a pin by port name first, then by physical pad
+// — so a netlist that names IC pins either by signal or by ball both bind.
+BsdlApplyReport apply_bsdl(Part *part, const BsdlModel &model);
+
+#endif // _BSDL_MODEL_HPP_