refactor: delete orphan declarations flagged by fallow (#949)

* ci: run fallow audit in lefthook pre-commit

Mirrors the same `fallow audit --base ... --fail-on-issues` check that
runs in CI, but locally against HEAD so issues surface at commit time
instead of after the push round-trip.

Scoped to `packages/**` source files via the glob — non-code edits
(README, docs, top-level configs) skip the hook entirely.

Measured locally: ~5s in parallel with the existing lint/format/typecheck
checks. Doesn't extend wall-clock time because typecheck (~11s) is the
long pole, and lefthook runs commands in parallel.

The default `--gate new-only` means inherited findings don't block the
commit — same gate behavior as CI, so local pre-commit and PR audit
agree.

* refactor: delete orphan declarations flagged by fallow

After fallow's auto-fix de-exports unused symbols, oxlint surfaces them
as no-unused-vars. This PR deletes those orphan declarations outright.

Biggest cleanup: studio/src/icons/SystemIcons.tsx shrinks from 132 to 57
lines — 33 unused icon wrappers and their phosphor-icon imports deleted.

Other deletions across 14 more files covering paired getter/setters,
helper functions, dead env constants, internal components with no
callers, and cascading unused imports.

Cascade-causing files held back for follow-up PRs: renderOrchestrator
barrel of captureCost re-exports, telemetry/portUtils/remote barrels,
Button.tsx + ui/index.ts (would orphan whole file), studioMotion
type re-exports.

Test plan: typecheck clean across 8 packages, oxlint + oxfmt clean,
fallow audit exit 0 (remaining findings inherited), cli + studio
vitest suites pass.

packages/studio/src/player/hooks/useTimelineSyncCallbacks.ts

@@ -18,7 +18,6 @@ import {
   findTimelineDomNodeForClip,
   createImplicitTimelineLayersFromDOM,
   buildStandaloneRootTimelineElement,
-  mergeTimelineElementsPreservingDowngrades,
   getTimelineElementSelector,
 } from "../lib/timelineDOM";
 import {
@@ -26,7 +25,6 @@ import {
   autoHealMissingCompositionIds,
   buildMissingCompositionElements,
 } from "../lib/timelineIframeHelpers";
-import { getTimelineElementIdentity } from "../lib/timelineElementHelpers";
 
 interface UseTimelineSyncCallbacksParams {
   iframeRef: React.RefObject<HTMLIFrameElement | null>;
@@ -288,7 +286,3 @@ export function useTimelineSyncCallbacks({
     onIframeLoad,
   };
 }
-
-// Re-export the merge helper so the hook can use it via this module (avoids
-// adding another import line to the already-large useTimelinePlayer.ts).
-export { mergeTimelineElementsPreservingDowngrades, getTimelineElementIdentity };