feat: syntax highlighting for TUI code blocks via tree-sitter

[bug-ops]

Summary

• Add tree-sitter-highlight based syntax coloring for fenced code blocks in TUI chat widget
• Support 6 languages: Rust, Python, JavaScript, JSON, TOML, Bash (with aliases rs/py/js/sh/shell)
• SyntaxHighlighter singleton via LazyLock with 11 capture categories (keyword, string, comment, function, type, number, operator, variable, attribute, punctuation, constant)
• Graceful fallback to flat theme.code_block style for unknown languages
• Migrated zeph-index grammar deps to workspace references (deduplication)

Closes #344, closes #345, closes #346, closes #347

Test plan

• cargo nextest run -p zeph-tui --lib — 89 tests pass (7 new)
• cargo nextest run --workspace --lib --bins — 1415 tests pass, 0 regressions
• cargo clippy -p zeph-tui -- -D warnings clean
• cargo +nightly fmt --check clean
• Security audit: cargo-deny clean, no unsafe code
• Performance: acceptable for MVP, highlight caching deferred
• Visual verification in TUI with multi-language code blocks

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.00000% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/zeph-tui/src/widgets/chat.rs	81.39%	8 Missing ⚠️
crates/zeph-tui/src/highlight.rs	99.03%	2 Missing ⚠️

@@            Coverage Diff             @@
##             main     #353      +/-   ##
==========================================
+ Coverage   79.18%   79.33%   +0.14%     
==========================================
  Files          99      100       +1     
  Lines       25428    25665     +237     
==========================================
+ Hits        20136    20361     +225     
- Misses       5292     5304      +12     

Files with missing lines	Coverage Δ
crates/zeph-tui/src/lib.rs	0.00% <ø> (ø)
crates/zeph-tui/src/theme.rs	100.00% <ø> (ø)
crates/zeph-tui/src/highlight.rs	99.03% <99.03%> (ø)
crates/zeph-tui/src/widgets/chat.rs	50.98% <81.39%> (+1.39%)	⬆️

... and 1 file with indirect coverage changes

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