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/STORYBOARD.md

@@ -0,0 +1,71 @@
+---
+format: 1920x1080
+message: "Ship a launch video in an afternoon"
+arc: Hook → Problem → Solution → Proof → CTA
+audience: indie devs on X
+voice: Rachel — 5 lines from SCRIPT.md
+---
+
+## Frame 1 — Hook
+
+- scene: Big type punches in on the beat
+- duration: 3s
+- poster: 2s
+- transition_in: cut
+- status: animated
+- voiceover: "Ship a launch video in an afternoon."
+- src: compositions/frames/01-hook.html
+
+Open cold on the promise. Big type punches in on the beat: "Ship a launch
+video in an afternoon." This is the thesis — everything after pays it off.
+
+## Frame 2 — The problem
+
+- scene: A 20-minute timer spins on a stack of rejected takes
+- duration: 4s
+- poster: 2.6s
+- transition_in: crossfade
+- status: animated
+- voiceover: "The old way? Prompt, wait twenty minutes, get something that misses."
+- src: compositions/frames/02-problem.html
+
+The old way: prompt, wait twenty minutes, get something that misses. A timer
+spins while a stack of rejected takes piles up. Establish the pain we remove.
+
+## Frame 3 — The feature in action
+
+- scene: Frames line up as a contact sheet
+- duration: 5s
+- poster: 3.3s
+- transition_in: crossfade
+- status: built
+- voiceover: "You lock the story first — then it animates."
+- src: compositions/frames/03-feature.html
+
+Cut to the storyboard view itself. Frames line up as a contact sheet and the
+narration says "...you lock the story first, then it animates." Show, don't tell.
+
+## Frame 4 — Proof
+
+- scene: A stat counts up to the payoff number
+- duration: 4s
+- poster: 2.6s
+- transition_in: wipe
+- status: built
+- voiceover: "Fewer re-renders. More finished videos."
+- src: compositions/frames/04-proof.html
+
+A real metric lands: fewer re-renders per finished video. Number counts up
+while a caption credits the storyboard-first flow.
+
+## Frame 5 — Call to action
+
+- scene: End card with the wordmark and a one-line CTA
+- duration: 3s
+- transition_in: crossfade
+- status: outline
+- voiceover: "Try it: npx hyperframes."
+- src: compositions/frames/05-cta.html
+
+End card, still an outline. "Try it: npx hyperframes" with the wordmark. Not
+built yet — this frame should render as an outline placeholder in the grid.