presence: fix stale peer selections after delete, select-all lag, ghost-peer removal (findings group Y)

- wasm core: CORE::onDocChanged() from both collab listeners (local commit AND
  remote apply) repaints peers' shapes from the live document + re-checks the
  local selection post-settle; PresenceStart registers the bridge listener
- wasm core: cursors on their own overlay trio; shapes repaint only when the
  non-cursor snapshot changes; new kicadCollabSetRemoteCursors (cursor-only
  update, PEER.id) in both TUs + merged editor + JSPI mutator allowlist
- presence.ts: size-aware trailing throttle for cursor/viewport publishes
  (128 KB/s budget) + parsed-peers memo; presence-kicad.ts: cursor-only push
  when the shape signature is unchanged
- gateway.ts: honor the `gone` control (removeAwarenessStates)
- specs: kicad stale-after-delete gate, web ghost-peer timing (+diag), unit
  select-all budget (pcbjam-shared → 4573a7c)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AScTR39aqyrY5i3ZFHmnMn

scripts/common/shims/jspi-scheduler.js

@@ -96,6 +96,7 @@
       "kicadCollabApply", "kicadCollabApplyItems",
       "kicadCollabSnapshot", "kicadCollabSnapshotItems",
       "kicadCollabPresenceStart", "kicadCollabSetRemote",
+      "kicadCollabSetRemoteCursors",
       "kicadCollabSetPins", "kicadCollabSetStyle",
       "kicadCollabSetViewport", "kicadCollabFitViewport",
       "kicadCollabReleaseSelection", "kicadSetColorTheme",