feat(index): Svelte support + TS arrow function extraction

[ajianaz]

Summary

Closes #384 — adds Svelte support and TS arrow function extraction to the AST indexer.

Phase 1: TS Arrow Functions

Previously, lexical_declaration and variable_declaration only captured ALL_CAPS constants. Now also captures:

• Arrow functions: export const handler = () => { ... }
• Function expressions: const cb = function doStuff() { ... }
• Call edges from arrow function bodies (via extract_calls_from_node)

Phase 2: Svelte Support

No tree-sitter-svelte dependency needed. Instead:

1. Extract <script> blocks from .svelte files via regex
2. Detect lang="ts" / lang="js" attribute (default: JS)
3. Parse extracted content using existing tree-sitter-typescript / tree-sitter-javascript grammar
4. Adjust line numbers with offset from script tag position
5. Handle both inline (<script>code</script>) and multi-line script blocks

What's NOT captured (expected)

• Svelte runes ($state, $derived, $props) — not valid TS/JS syntax
• Component name from filename — AST parser works on code, not file names
• Template calls (onclick={handler}) — HTML template is not parsed
• Nested arrow functions inside callbacks — scope limitation

Test Results

• 7 new unit tests: all pass
• Updated 1 existing Svelte test for AST behavior
• Full suite: 739/740 pass (1 pre-existing Ruby test failure, unrelated)
• Real-world validated on cora-api SvelteKit project:
  • 86 .svelte files indexed
  • Functions extracted with correct line numbers
  • Svelte symbols appear in brain search
  • Arrow functions in .ts files captured (SvelteKit load functions)

Files Changed

• src/index/ast.rs — extract_svelte(), arrow function support in extract_typescript(), 7 tests
• src/index/extract.rs — Svelte bypass in 3 get_language() gates, updated Svelte test
• Cargo.toml — version bump to 0.8.3