Altium import, nets, canonical pins, component kinds, set/$var, scrollback, source modal.

Major additions, all wired end-to-end with doctest coverage:

- Altium netlist importer (`imports/import_altium.{hpp,cpp}`): two-pass
  parser for `[ ]` parts and `( )` signals; `System::Load` no longer has
  the IMPORT_ALTIUM hole.
- `duplicate <src> <dst>` deep-copies a module (signals, parts, pins,
  rewired signals); connections excluded by design.
- Nets (`system/nets.{hpp,cpp}`): BFS over `Connection::pin_map` to
  return the transitive (Module, Signal) closure. `verify` extended with
  a second pass flagging Power↔GndShield inconsistencies in bridged
  nets; new `net <module> <signal>` command for inspection.
- Canonical pin names (`system/pin_name.{hpp,cpp}`): zero-padded digit
  suffix lets A1 ↔ A001 pair via `IdentityTransform` and
  `CheckIdentityCompatible` without losing the imported notation.
- Component classification (`system/component_kind.{hpp,cpp}`):
  `Part::kind` inferred at construction from the reference-designator
  prefix (longest-match: LED/TP/SW/FB/MK/MP/MH/HS/RA/RN/RP/RV first,
  then R/C/L/F/D/Q/U/J/P/Y/X/S).
- Identity wiring tolerance: `CheckIdentityCompatible` accepts the
  subset case (typical when one importer drops NC pins, e.g. Altium)
  and surfaces orphans as an info string. `FillIdentityNCs`
  materialises orphan canonical positions as NC pins on the missing
  side at connect time.
- Connector layout preparation: `pin_layout(kind)` and
  `FillPartFromLayout(part, kind)` stubs in `pin_role`, called from
  `set-type`. Empty today; populate alongside `vpx_3u_role`.
- TUI scrollback: PageUp/PageDown step 10 lines, Home/End jump to
  ends; `Print()` snaps back to the tail.
- `set <name> <value>` declares session variables; `$name` / `${name}`
  expanded inside `Finalize` between canonical-form recording and the
  action call — history and script-save preserve `$var` references.
- Long `source` scripts now show a centred "Computing…" modal with a
  N/M progress counter. Driven by a ticker thread that posts one
  paced `Event::Special` per processed line, ack'd by the main thread,
  so heavy lines don't backlog ticks and freeze the counter.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/tui/tui.cpp

@@ -9,7 +9,10 @@
 using namespace ftxui;
 
 Tui::Tui()
-    : cursor_pos(0), history_idx(-1), quit(false), in_source(false),
+    : cursor_pos(0), history_idx(-1), scroll_offset(0), quit(false), in_source(false),
+      loading(false), tick_in_flight(false),
+      loading_idx(0), loading_executed(0), loading_lineno(0),
+      loading_prev_in_source(false), screen_ptr(nullptr),
       screen_idx(0),
       search_types{"parts", "signals"},
       search_module_idx(0), search_type_idx(0), search_focus_idx(0),
@@ -31,6 +34,7 @@ Tui::~Tui() = default;
 
 void Tui::Run() {
     auto screen = ScreenInteractive::Fullscreen();
+    screen_ptr = &screen;
 
     auto main_screen    = BuildMainScreen(screen);
     auto search_screen  = BuildSearchScreen();
@@ -69,7 +73,12 @@ void Tui::Run() {
             return false;
 
         default:  // main
+            if (e == Event::Special("\x02tick")) { ProcessNextSourceLine(); return true; }
             if (e == Event::Escape && !pending.empty()) { CancelPending(); return true; }
+            if (e == Event::PageUp)   { scroll_offset += 10; return true; }
+            if (e == Event::PageDown) { scroll_offset = std::max(0, scroll_offset - 10); return true; }
+            if (e == Event::Home)     { scroll_offset = (int)output.size(); return true; }
+            if (e == Event::End)      { scroll_offset = 0; return true; }
             if (e == Event::ArrowUp || e == Event::ArrowDown) {
                 if (pending.empty()) {
                     if (e == Event::ArrowUp) HistoryUp();