feat(highlight): user-supplied sublime-syntax via syntaxes/ next to the deck

[mizzy]

Closes #116

Summary

• Introduces peitho_core::highlight::Highlighter — a newtype owning syntect::parsing::SyntaxSet, threaded through parse_markdown and render_deck as &Highlighter. The old OnceLock<SyntaxSet> global and the free validate_language / highlight_html are deleted; no free function on highlight:: takes a language token without a &Highlighter anymore, so the "default set only" path is unrepresentable at the API surface.
• Highlighter::with_user_dir(dir) augments the built-in set with *.sublime-syntax files under dir. A malformed file becomes a line-numbered BuildError (silent drop forbidden). An empty syntaxes/ behaves like defaults.
• The CLI discovers syntaxes/ next to the deck by convention (same shape as layouts/ / css/) — no CLI flag added, per author decision. peitho build --watch rebuilds on .sublime-syntax changes.

Three lenses

• Long-term — any DSL (Carina .crn, TypeScript, custom repo languages) gains highlighting without peitho having to know about it. Also unblocks a workable answer for Unknown-language help text suggests 'ts', but syntect's default set has no TypeScript #105 (TypeScript missing from defaults): drop a TS .sublime-syntax in syntaxes/.
• Type safety — Highlighter is the only way to reach find_syntax_by_token. A caller cannot forget user syntaxes; the type requires an instance.
• Root cause — the reason a .crn block was rejected before was not "Carina is missing" (symptom) but "the syntax set is baked in at process init with no injection point" (root). The one upstream fix threads a Highlighter through the pipeline.

All three lenses agreed unambiguously, so this shape was chosen without ambiguity.

What is NOT in this PR (by explicit plan decision)

• No CLI flag (--syntaxes) — convention-only per author.
• No frontmatter key (belongs to Deck frontmatter: move deck-intrinsic options (layouts, css, ...) from CLI into frontmatter #62's migration).
• No bundled HCL/TypeScript syntax (Option 2 in the issue was ruled out).
• No Unknown-language help text suggests 'ts', but syntect's default set has no TypeScript #105 help-text change — sibling problem, out of scope.

Test plan

• cargo test --workspace (x3)
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all --check
• git diff --exit-code bindings/
• cd packages/peitho-present && npm run build && npm test && npm run typecheck
• git diff --exit-code packages/peitho-present/dist/shell.js
• Headline test: build_artifacts_uses_syntaxes_dir_next_to_the_deck — a deck with a ```carina block plus syntaxes/carina.sublime-syntax next to it builds and produces hl-* spans.
• Reversal test: user_dir_validates_carina_and_defaults_reject_it — same carina token is a build error under Highlighter::defaults().

Design record: docs/plans/2026-07-05-user-syntaxes.md.

🤖 Generated with Claude Code