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/compositions/frames/02-problem.html

@@ -0,0 +1,62 @@
+<template id="frame-02-problem-template">
+  <div
+    data-composition-id="frame-02-problem"
+    data-width="1920"
+    data-height="1080"
+    data-duration="4"
+  >
+    <div class="canvas">
+      <div class="spinner"></div>
+      <div class="timer">20:00</div>
+      <div class="label">prompt · wait · miss · repeat</div>
+    </div>
+
+    <style>
+      [data-composition-id="frame-02-problem"] .canvas {
+        width: 1920px;
+        height: 1080px;
+        background: #0c0a14;
+        display: flex;
+        flex-direction: column;
+        align-items: center;
+        justify-content: center;
+        gap: 40px;
+        font-family: "Inter", system-ui, sans-serif;
+        overflow: hidden;
+      }
+      [data-composition-id="frame-02-problem"] .spinner {
+        width: 180px;
+        height: 180px;
+        border-radius: 50%;
+        border: 14px solid #2a2342;
+        border-top-color: #8b7bff;
+      }
+      [data-composition-id="frame-02-problem"] .timer {
+        color: #fff;
+        font-size: 96px;
+        font-weight: 700;
+        font-variant-numeric: tabular-nums;
+        letter-spacing: 0.02em;
+      }
+      [data-composition-id="frame-02-problem"] .label {
+        color: #6f6790;
+        font-size: 34px;
+        letter-spacing: 0.08em;
+        text-transform: uppercase;
+        opacity: 0;
+      }
+    </style>
+
+    <script src="https://cdn.jsdelivr.net/npm/gsap@3.14.2/dist/gsap.min.js"></script>
+    <script>
+      (function () {
+        const id = '[data-composition-id="frame-02-problem"]';
+        const tl = gsap.timeline({ paused: true });
+        tl.to(id + " .spinner", { rotation: 720, duration: 4, ease: "none" }, 0);
+        tl.to(id + " .label", { opacity: 1, duration: 0.5, ease: "power2.out" }, 0.8);
+        window.__timelines = window.__timelines || {};
+        window.__timelines["frame-02-problem"] = tl;
+      })();
+    </script>
+  </div>
+</template>