M23-A: SyntaxTheme and highlight.rs scaffolding

[bug-ops]

Parent: #344

Scope

• Add tree-sitter-highlight to workspace deps
• Add tree-sitter + grammar crates to zeph-tui/Cargo.toml behind feature flags
• Create SyntaxTheme struct in theme.rs with styles for: keyword, string, comment, function, type, number, operator, variable, attribute, punctuation, constant
• Add pub syntax: SyntaxTheme field to Theme
• Create crates/zeph-tui/src/highlight.rs with SyntaxHighlighter struct
• SyntaxHighlighter::new() registers available grammars
• SyntaxHighlighter::highlight(lang, code, theme) -> Option<Vec<Span>>
• LazyLock<SyntaxHighlighter> at module level
• Add mod highlight to lib.rs

Acceptance criteria

• SyntaxHighlighter compiles and initializes without panic
• At least Rust grammar registered and produces highlighted spans
• Fallback returns None for unknown languages
• Feature flags match zeph-index pattern (lang-rust, lang-python, lang-js, lang-config)