feat(studio): add storyboard manifest contract, parser, and read API (#1528)

First PR in the Studio storyboarding stack. Establishes the parseable
contract the storyboard UI reads from; no UI yet.

- core/storyboard: StoryboardManifest/Frame/Globals types + a lenient
  STORYBOARD.md parser (frontmatter + status/src/duration/transition_in,
  freeform narrative tolerated, never throws, records warnings). Exposed as
  @hyperframes/core/storyboard (browser-safe).
- studio-api: GET /projects/:id/storyboard returns the normalized manifest
  with per-frame srcExists; missing file -> exists:false, not 404.
- fixture: packages/studio/fixtures/storyboard-sample for dogfooding the
  storyboard view in later PRs (built/animated frames + one outline).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/studio/fixtures/storyboard-sample/README.md

@@ -0,0 +1,26 @@
+# storyboard-sample (Studio fixture)
+
+A small, self-contained project for dogfooding the Studio **Storyboard** view.
+Not a registry catalog item — it lives here purely as test content for the
+storyboard UI/UX work.
+
+It exercises the storyboard contract end to end:
+
+- `STORYBOARD.md` — structured manifest (frontmatter + 5 frames) in the canonical
+  format the parser reads.
+- `compositions/frames/0{1..4}-*.html` — live HTML frame sub-compositions
+  (`built` / `animated` statuses) the contact-sheet tiles render.
+- Frame 5 (`05-cta.html`) is intentionally **absent** and `status: outline`, so
+  the grid has an outline placeholder to render.
+
+Preview the storyboard view (flag-gated):
+
+```bash
+VITE_STUDIO_ENABLE_STORYBOARD=1 npx hyperframes preview packages/studio/fixtures/storyboard-sample
+```
+
+Inspect just the parsed manifest the Studio consumes:
+
+```bash
+curl localhost:<port>/api/projects/<id>/storyboard | jq
+```