chore: reorganize fixtures and harden heading PNG tests

[rrbe]

Summary

• Consolidate 7 overlapping markdown fixtures into 4 purpose-named ones; new fixtures/specialized/ subdir houses narrow single-purpose fixtures (headings basics, headings emoji, large stress)
• Split the bundled heading PNG integration test into 4 focused assertions, and enforce display-form a=T on every emitted APC frame (the old check only looked for one occurrence in stdout, so a stray a=t would slip through and render blank in cat mode)
• New headings-emoji.md fixture + emoji_headings_emit_decodable_nonblank_pngs test guards against emoji font fallback regressions silently producing blank glyphs
• Add docs/TERMINAL_PROTOCOLS.md explaining the ANSI / Kitty graphics protocol layering, how fixtures/expected/*.ansi captures both, and how strip_kitty_images / extract_kitty_frames work

Fixture layout after this change

fixtures/
├── supported-syntax.md       # current + roadmap syntax (HTML, images, wrap-indent, metadata, TOC)
├── unsupported-syntax.md     # out-of-scope features only
├── links/                    # multi-file TUI link-nav fixture
├── specialized/              # narrow single-purpose fixtures
│   ├── headings-basic.md     # minimal H1/H2/H3 for PNG height test
│   ├── headings-emoji.md     # emoji + CJK headings, font-fallback regression guard
│   └── large.md              # ~15k-line stress fixture (regenerate with scripts/gen-large-fixture.sh)
└── expected/                 # snapshot baselines
    ├── supported-syntax.ansi
    └── unsupported-syntax.ansi

Snapshot baselines for the two consolidated fixtures were regenerated. TODO.md's "整理测试用的 markdown 文件" item is checked off.

Test plan

• make check passes (fmt-check + clippy + tests, including the 4 new heading tests and 2 snapshot tests)
• cargo run -- --cat fixtures/specialized/large.md exits cleanly on the stress fixture
• cargo run -- --tui fixtures/links/index.md still navigates link-following correctly (manual)
• cargo run -- --tui fixtures/supported-syntax.md renders all 12 sections without panic (manual)

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