test(engine): add ffprobe-unavailable fallback regression tests

[vanceingalls]

Summary

Mock node:child_process.spawn to surface ENOENT and verify ffprobe's three callers behave correctly when ffprobe is missing.

Why

Chunk 9B of plans/hdr-followups.md. The PNG cICP fallback in extractMediaMetadata was added to support environments without ffprobe, but no test pinned the behavior — silently regressing it would break HDR image support on any system without ffprobe installed.

What changed

packages/engine/src/utils/ffprobe.test.ts: mocks child_process.spawn to surface ENOENT and asserts:

• extractMediaMetadata falls back to PNG cICP metadata for image inputs.
• extractMediaMetadata rethrows for non-image inputs lacking a still-image fallback.
• extractAudioMetadata + analyzeKeyframeIntervals propagate the install-hint error verbatim.

Test plan

• All new tests pass.
• No production code changes — pure regression coverage of existing fallback behavior.

Stack

Chunk 9B of plans/hdr-followups.md. Test-only change, independent of all other chunks.

[vanceingalls]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• docs(engine): document __name polyfill and add regression test #385
• perf(producer): cache transfer-converted hdr image buffers per render job #384
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• test(producer): extract frameDirMaxIndexCache to its own module and pin cross-job isolation #381
• test(engine): cover spawnStreamingEncoder lifecycle and cleanup paths #380
• test(engine): add ffprobe-unavailable fallback regression tests #379 👈 (View in Graphite)
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• refactor(shader-transitions): extract DEFAULT_DURATION and DEFAULT_EASE constants #367
• refactor(types): tighten type safety, dedupe HfTransitionMeta, prune dead LUT export #366
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• main

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