fix(video_compose): derive the Remotion theme from real playbook keys

[bbudaedu]

The defect

VideoCompose._build_theme_from_playbook documents itself as:

Derive a Remotion ThemeConfig from a playbook's actual color values. Instead of passing a playbook name and hoping Remotion has a matching preset, we read the playbook YAML and extract concrete colors/fonts.

Three of the keys it reads are not in schemas/styles/playbook.schema.json, so each silently fell back to a hardcoded default for every playbook:

Read	Schema key	Consequence
typo.get("heading")	headings	every playbook rendered its headings in Inter
palette.get("muted_text")	muted	mutedTextColor was always #6B7280
motion.get("pace")	identity.pace	no playbook's declared pace ever reached the spring config

Before → after:

flat-motion-graphics  headingFont  Inter    -> Space Grotesk
                      muted        #6B7280  -> #64748B
                      spring       20/120 @0.4s -> 12/80 @0.3s   (pace: fast)
minimalist-diagram    headingFont  Inter    -> IBM Plex Sans
anime-ghibli          headingFont  Inter    -> Noto Serif JP

This is the Remotion counterpart of the HyperFrames style-bridge defect in #306. That bridge lives in lib/hyperframes_style_bridge.py and is untouched here, as are the open PRs against it (#296, #307).

Note on ordering

Root.tsx's resolveTheme currently prefers a named preset over themeConfig, and four of the five playbook names collide with Remotion preset names. So the derived theme was being discarded exactly where these defaults would have shown — which masked this bug.

Fixing resolveTheme first would have made every video lose its heading typeface, muted color and motion pacing in one step. This PR has to land first.

Deliberately left alone

• palette.get("surface", bg) also names a key the schema does not define, but the fallback to the background color is an explicit, sensible default rather than a dropped value. Giving playbooks a real surface color would be a schema addition, not a bug fix.
• The pace branch still handles only fast and slow, so deliberate and rapid land on the moderate default. Extending that mapping is a motion tuning decision, not part of reading the right key.

Coverage

tests/contracts/test_remotion_theme_playbook_keys.py takes its expectations from the playbook YAML rather than a hardcoded table, so a playbook that changes its typeface keeps the test honest rather than making it stale.

Verification

• 4 cases fail on the unfixed tree; all pass after.
• Full suite: 964 → 977 passed, 13 skipped, no regressions.
• make lint: passed.

The 3 skips are anime-ghibli, which cannot yield a theme on current main for the reason #468 fixes; they start running once that lands.

Independent of #468, #469, #472 and #473 — no overlapping files.

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