Manifest-based binary resolution (glibc/musl aware, sha256-verified) + cross-platform encode tests

[zackees]

Implements the client-side foundation of #20 and the cross-platform encode-test guarantee, without disturbing any existing install.

What this PR does

Backward compatibility (hard requirement)

• The legacy ffmpeg_bins per-platform zip URLs are untouched and remain the default source. Old installs — which ship their own run.py — keep resolving to the exact same URLs. This PR only changes behavior for new/updated installs, and even then falls back to the legacy URL on any failure.

Manifest-based resolution (static_ffmpeg/manifest.py)

• Detects a platform tuple {os, arch, libc}. Linux distinguishes glibc vs musl (via platform.libc_ver() → ldd → musl-loader probe), so we can ship both a glibc-2.17 baseline and a musl build per arch.
• Resolves the tuple against a zackees/manifest.json Catalog (channel → release → per-platform asset).
• Verifies sha256 after download; a mismatch deletes the file and raises.
• Best-effort & disabled by default: returns None (→ legacy fallback) when the manifest is unset, unreachable, malformed, or missing the platform. Opt in with STATIC_FFMPEG_MANIFEST_URL; flip the default once ffmpeg-bins2 + CDN are live.

Tests

• tests/test_encode.py: a real small encode — synthesizes 1s of video+audio via lavfi, encodes to H.264/AAC mp4, and probes it back asserting a video stream. Rides all five existing per-platform test workflows (Win x64, macOS x64/ARM, Ubuntu x64/ARM), so every platform is guaranteed to actually encode, not just print -version.
• tests/test_manifest.py: offline resolver unit tests (glibc/musl split, sha256 verify, disabled→None) + an 8-target contract test against manifest.example.json.

Docs

• manifest.example.json documents the exact catalog contract for all 8 issue targets (win x64/arm, mac x64/arm, linux x64/arm × glibc/musl), with CDN-fronted urls[].
• README section on resolution + the backward-compat guarantee.

Verified locally

• pytest tests/ → 15 passed, 1 skipped (Windows-only permission test).
• flake8 clean · mypy clean · pylint 10.00/10.

Out of scope (tracked follow-ups on #20)

These are operational/infra steps that can't be completed from a coding session and stay tracked on the issue:

• Building FFmpeg (full features incl. nonfree) via zackees/forge for all 8 targets.
• Creating ffmpeg-bins2, storing binaries in LFS, and serving them via the CDN-fronted www site.
• Publishing the real manifest.json (real versions/sizes/sha256/urls) and flipping DEFAULT_MANIFEST_URL on.
• New test workflows for Windows-ARM64 and Linux-musl (need the new binaries first).

Closes part of #20.

🤖 Generated with Claude Code