M23: Syntax highlighting for TUI code blocks

[bug-ops]

Tree-sitter-based syntax highlighting for fenced code blocks in the TUI chat widget.

Current state: Code blocks render with a single flat color (theme.code_block).
Goal: Language-aware syntax coloring using tree-sitter grammars already in the workspace.

Architecture

• SyntaxHighlighter in crates/zeph-tui/src/highlight.rs wraps tree-sitter-highlight
• SyntaxTheme maps tree-sitter capture names (keyword, string, comment, etc.) to ratatui styles
• MdRenderer calls highlighter when rendering code block text with a known language tag
• LazyLock<SyntaxHighlighter> for zero-cost initialization
• Feature-gated grammars: lang-rust, lang-python, lang-js, lang-config

Languages (MVP)

Rust, Python, JavaScript, TypeScript, JSON, TOML, Bash

Sub-issues

• M23-A: SyntaxTheme and highlight.rs scaffolding #345 Phase A: SyntaxTheme + highlight.rs scaffolding
• M23-B: Integrate SyntaxHighlighter into MdRenderer #346 Phase B: Integrate into MdRenderer
• M23-C: Add highlight queries for MVP languages #347 Phase C: Highlight queries for all MVP languages

Plan

.local/plan/m23-syntax-highlighting.md