feat: parse and render markdown frontmatter metadata

[rrbe]

Summary

• YAML (---) and TOML (+++) frontmatter blocks at the top of a doc are now parsed via pulldown-cmark's metadata extensions and never leak into body content (the long-standing setext-H2-rasterized-as-PNG bug at the top of any file with frontmatter is gone).
• --cat renders a single dim summary line: · metadata · title=…, author=…, … (truncated to terminal width).
• TUI shows the same folded line by default and adds the m key to expand it into an inline key/value box. Default state: folded.
• Single config knob in ~/.termdown/config.toml:

  [metadata]
  show = true   # false hides metadata in both cat and TUI

  Parsing always runs (so frontmatter can't leak); show only gates display.

Design

Full rationale, alternatives considered, and rejected designs in docs/adr/0001-metadata-block-handling.md. Key choices:

• Heuristic line-based parser, not a real YAML/TOML library — termdown's output is a 1-line summary, not a validator.
• Default folded, not glow-style "completely hidden" — readers of agent-skill / Hugo / Jekyll files care about title/author/date enough to deserve a discoverable summary, but not enough to deserve permanent screen real estate.
• m keybinding is new; added to the ? help screen.

A glossary lives in the new CONTEXT.md for future maintainers.

Test plan

• make check clean (fmt, clippy -D warnings, full test suite).
• New focused fixtures under fixtures/specialized/:
  • metadata-yaml.md — happy path for YAML.
  • metadata-toml.md — happy path for TOML.
  • metadata-malformed.md — nested + multi-line YAML, exercises the heuristic's edge behavior.
  • metadata-none.md — regression guard: mid-doc --- must stay a horizontal rule.
• Existing fixtures/expected/supported-syntax.ansi updated (the top-of-doc setext-H2 PNG noise → one dim summary line). Eyeballed the diff.
• Unit tests for the heuristic in src/frontmatter.rs (empty block, multi-line continuation, value containing :, fallback path).
• Cat smoke test on a synthetic 6-field YAML probe — output matches design.
• Manual TUI verification: open a doc with frontmatter, press m to expand/collapse. TODO before merge.

Drive-by

tests/cli.rs::TempMarkdownFile used pid + nanos for uniqueness — with the four new snapshot tests pushing parallelism higher, two test threads occasionally landed on the same nanosecond and shared a filename. Added a process-local atomic counter; flake is gone (5 consecutive clean cargo test runs verified).

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