fix: add media rendering guardrails to prevent silent failures (#112)

## Summary

- **Lint rules** catch media elements missing `id` (renderer silently skips them), missing `src`, `preload="none"` (blocks renderer), and video nested in timed divs (freezes playback). Upgraded `video_nested_in_timed_element` from warning to error.
- **Compiler** strips `preload="none"` from media during compilation. Runs parallel, cached keyframe interval analysis via ffprobe — warns on sparse keyframes (>2s) that cause seek failures and audio/video desync. Suggested ffmpeg command preserves audio (`-c:a copy`).
- **Pre-render lint** lints `index.html` + all `compositions/*.html` sub-compositions before render via shared `lintProject()` helper. Warns by default; `--strict` blocks on errors, `--strict-all` blocks on errors + warnings.
- **Render orchestrator** logs a hint to retry with `--workers 1` when parallel capture times out on video-heavy compositions.
- **Refactor**: extracted `runFfprobe()` + `parseProbeJson()` helpers to deduplicate ~80 lines of spawn boilerplate across 3 ffprobe functions. Extracted `shouldBlockRender()` so strict flag tests exercise production code. Shared `lintProject()` used by both `lint` and `render` commands.

## Context

Discovered during a real composition build session where:
1. `<audio>` without `id` rendered silently (preview worked fine because runtime queries `[data-start]`, but renderer queries `[id][src]`)
2. `<video>` inside timed `<div>` froze on first frame
3. `preload="none"` caused 45s renderer timeout
4. YouTube clips with sparse keyframes from `yt-dlp --download-sections` caused audio/video desync
5. Parallel workers timed out on video-heavy compositions

## Test plan

- [x] Core: 365/365 tests passing (5 new lint tests)
- [x] Engine: 24/24 tests passing
- [x] CLI: 14/14 tests passing (7 lintProject + 7 shouldBlockRender)
- [x] Lint + format hooks pass
- [ ] Manual: create a composition with `<audio data-start="0" src="test.wav">` (no id) — verify `npx hyperframes lint` catches it
- [ ] Manual: run `npx hyperframes render --strict` with lint errors — verify it blocks
- [ ] Manual: run `npx hyperframes render --strict-all` with lint warnings — verify it blocks

packages/core/src/lint/hyperframeLinter.ts

@@ -379,8 +379,8 @@ export function lintHyperframeHtml(
         if (!parentClosePattern.test(between)) {
           pushFinding({
             code: "video_nested_in_timed_element",
-            severity: "warning",
-            message: `<video> with data-start appears to be nested inside <${parent.name}${parent.id ? ` id="${parent.id}"` : ""}> which also has data-start. This can break media sync.`,
+            severity: "error",
+            message: `<video> with data-start is nested inside <${parent.name}${parent.id ? ` id="${parent.id}"` : ""}> which also has data-start. The framework cannot manage playback of nested media — video will be FROZEN in renders.`,
             elementId: readAttr(tag.raw, "id") || undefined,
             fixHint:
               "Move the <video> to be a direct child of the stage, or remove data-start from the wrapper div (use it as a non-timed visual container).",
@@ -459,6 +459,46 @@ export function lintHyperframeHtml(
     }
   }
 
+  // #3.8: Media element checks — missing id, missing src, preload="none"
+  // The runtime discovers media via querySelectorAll("video[data-start]") which
+  // works fine for preview. But the renderer uses querySelectorAll("video[id][src]")
+  // — without id, elements are silently skipped (no audio, frozen video).
+  for (const tag of tags) {
+    if (tag.name !== "video" && tag.name !== "audio") continue;
+    const hasDataStart = readAttr(tag.raw, "data-start");
+    const hasId = readAttr(tag.raw, "id");
+    const hasSrc = readAttr(tag.raw, "src");
+    if (hasDataStart && !hasId) {
+      pushFinding({
+        code: "media_missing_id",
+        severity: "error",
+        message: `<${tag.name}> has data-start but no id attribute. The renderer requires id to discover media elements — this ${tag.name === "audio" ? "audio will be SILENT" : "video will be FROZEN"} in renders.`,
+        fixHint: `Add a unique id attribute: <${tag.name} id="my-${tag.name}" ...>`,
+        snippet: truncateSnippet(tag.raw),
+      });
+    }
+    if (hasDataStart && hasId && !hasSrc) {
+      pushFinding({
+        code: "media_missing_src",
+        severity: "error",
+        message: `<${tag.name} id="${hasId}"> has data-start but no src attribute. The renderer cannot load this media.`,
+        elementId: hasId,
+        fixHint: `Add a src attribute to the <${tag.name}> element directly. If using <source> children, the renderer still requires src on the parent element.`,
+        snippet: truncateSnippet(tag.raw),
+      });
+    }
+    if (readAttr(tag.raw, "preload") === "none") {
+      pushFinding({
+        code: "media_preload_none",
+        severity: "warning",
+        message: `<${tag.name}${hasId ? ` id="${hasId}"` : ""}> has preload="none" which prevents the renderer from loading this media. The compiler strips it for renders, but preview may also have issues.`,
+        elementId: hasId || undefined,
+        fixHint: `Remove preload="none" or change to preload="auto". The framework manages media loading.`,
+        snippet: truncateSnippet(tag.raw),
+      });
+    }
+  }
+
   // #4: Timed element missing visibility:hidden (no class="clip" or equivalent)
   // Skip: elements with data-composition-id (managed by runtime), elements with
   // opacity:0 in style (will be animated in by GSAP), and composition host elements.