feat: viewer panels — layer selector + selection inspector (viewer-panels)

Canvas-only sessions (read-only viewers, hide-UI editors) get two
floating, draggable, closable panels standing in for the chrome-hidden
wx panes, opened from the overlay menu's View section:

- wasm/bindings: layer bridge — kicadLayersGetState/SetVisible/SetActive
  (bodies mirror the compiled-in-but-unreachable IPC handlers, applies
  on the coroutine lane, fresh state pushed to
  window.kicadCollab.onLayersState); setters join the jspi-scheduler
  mutator lane.
- standalone: LayerPanel + SelectionInspector on the comments-panel
  shell conventions (useDraggablePanel, collapse, persisted state);
  local-selection store fed from presence's onSelection in edit
  sessions and bindLocalSelectionFeed for read-only viewers (with a
  bounded post-gesture pull burst — clarify-menu selections produce no
  canvas event); pure item-summary extraction + unit tests.
- kicad submodule: read-only selection unlock (selection live for
  inspection; point editors + RMB context menus stay locked).
- tests/web: read-only spec updated — viewer click selects (or pops the
  clarify list), RMB context menu suppressed with writer positive
  control, Delete still swallowed; new viewer-panels test (eye toggle
  round-trip, active layer, panel drag, inspector rows from a real
  canvas click).

Record: docs/features/read-only-viewer/0002-viewer-panels.md (root).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011gJ3M1RpeZLeNUUj8jKC4h

scripts/common/shims/jspi-scheduler.js

@@ -100,6 +100,7 @@
       "kicadCollabSetViewport", "kicadCollabFitViewport",
       "kicadCollabReleaseSelection", "kicadSetColorTheme",
       "kicadSaveBoard", "kicadSaveSchematic", "kicadSaveDrawingSheet",
+      "kicadLayersSetVisible", "kicadLayersSetActive",
     ],
     mutatorQueue: [],
     mutatorsWrapped: 0,