chore: Update constants for FFmpeg 8.1

[ldm0]

No description provided.

[Copilot]

Pull request overview

Updates the crate’s hand-written FFmpeg constants and feature gates to align with newer upstream releases (notably the FFmpeg 8.x / 8.1 era), and adds internal documentation to standardize future constant-audit workflows.

Changes:

• Added new pixel-format endian aliases in src/avutil/pixfmt.rs, including new FFmpeg 8-gated formats and narrower gates using the new ffmpeg6_1 boundary.
• Added AVERROR_HTTP_TOO_MANY_REQUESTS behind ffmpeg7_1.
• Introduced ffmpeg6_1 and ffmpeg8_1 feature boundaries in Cargo.toml, plus a “ffmpeg-constant-sync” skill/doc set for repeatable audits.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/avutil/pixfmt.rs	Adds/adjusts endian alias constants and refines version feature gates.
src/avutil/error.rs	Adds a new HTTP error constant with a version gate.
Cargo.toml	Adds ffmpeg6_1 and ffmpeg8_1 feature boundaries and adjusts the feature inheritance chain.
skills/ffmpeg-constant-sync/SKILL.md	Documents a repeatable workflow for syncing constants against upstream FFmpeg tags.
skills/ffmpeg-constant-sync/references/tag-audit-commands.md	Provides concrete commands for tag diffs and first-introduction checks.
skills/ffmpeg-constant-sync/agents/openai.yaml	Registers the skill for agent-driven auditing prompts.
.gitignore	Ignores the local /.codex directory.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.