feat(collab): cross-app selection — eeschema symbol ⇄ pcbnew footprint ghost highlight (collab-presence 0006)

Selecting a symbol in eeschema ghost-highlights the linked footprint(s) in
every pcbnew tab of the project, and vice versa — across users AND one
user's own two tabs. Native KIWAY cross-probe is inert in WASM (one frame
per page); this rides the presence layer instead.

- cross-app.ts: project-wide awareness-only room (presenceRoomId), publishes
  full PresenceState at selection rate (cursor always null); peers() = other-
  TOOL clients incl. own user's other tabs; window.__pcbjamCrossApp test handle
- presence-kicad.ts: parseSelectionEmit (bare array | {uuids,fpPaths}),
  xselFromPeerState (pcbnew paths → symbol uuids; eeschema uuids verbatim),
  cross peers appended to the kicadCollabSetRemote snapshot as
  {id "<user>#x<client>", name "<user> · sch|pcb", xsel}
- C++ (zero fork changes): pcbnew emits {uuids, fpPaths} (FOOTPRINT::GetPath)
  and ghost-renders xsel via path-tail suffix scan; eeschema resolves xsel via
  ResolveItem gated to the CURRENT sheet (xsel arrives project-wide, unlike
  room-scoped selections); ghostStyle = alphas × xselAlphaScale (0.55, tuner-
  patchable); new exports kicadCollabGetSelectionFull / TestGetCrossMapped /
  TestSelectComponent (skips power symbols — PWR_FLAG has no footprint) +
  merged-image dispatch
- tests: presence suites extended (payload shape, ghost render pixel tests,
  13/13) + new two-tab tests/web/cross-probe.spec.ts (passing vs real
  partykit); eeschema pixel compares now target the #glcanvas-* GAL panel
  (the whole-window #canvas compare flaked on the auto-dismissing version
  infobar — also fixes the long-known presence-eeschema restore flake);
  cross-app + presence-kicad vitest suites

Spec: docs/features/collab-presence/0006 (closed repo).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lg5jwWuhFH5dL8hEcBDuP2

wasm/bindings/collab_presence_style.h

@@ -82,6 +82,13 @@ struct STYLE
     double pinFillAlpha    = 1.0;
     double pinResolvedAlpha = 0.3;
 
+    // ── cross-app "ghost" selection (0006) ────────────────────────────────
+    // A peer's selection in the OTHER editor (eeschema symbol ⇄ pcbnew
+    // footprint) renders with the normal selection shape but its stroke/fill
+    // alphas scaled down, so a cross-probe highlight reads distinctly softer
+    // than a direct same-document selection.
+    double xselAlphaScale = 0.55;
+
 };
 
 /**
@@ -157,6 +164,18 @@ inline void patchStyle( STYLE& aStyle, const json& j )
     aStyle.pinRingAlpha     = j.value( "pinRingAlpha", aStyle.pinRingAlpha );
     aStyle.pinFillAlpha     = j.value( "pinFillAlpha", aStyle.pinFillAlpha );
     aStyle.pinResolvedAlpha = j.value( "pinResolvedAlpha", aStyle.pinResolvedAlpha );
+
+    aStyle.xselAlphaScale = j.value( "xselAlphaScale", aStyle.xselAlphaScale );
+}
+
+/** The style a cross-app (0006) ghost selection draws with: the given style
+ *  with its selection stroke/fill alphas scaled by `xselAlphaScale`. */
+inline STYLE ghostStyle( const STYLE& aStyle )
+{
+    STYLE g = aStyle;
+    g.selStrokeAlpha *= aStyle.xselAlphaScale;
+    g.selFillAlpha   *= aStyle.xselAlphaScale;
+    return g;
 }
 
 /** The color a peer renders with under this style (fixed > palette-by-name-hash