feat(extraction+resolution): Astro support — frontmatter/template extraction + src/pages routes (#768) (#815)

.astro files were not indexed at all, leaving a typical Astro site mostly
invisible to search/impact/explore. New AstroExtractor (Svelte/Vue SFC
pattern): component node per file, TS frontmatter + <script> blocks
delegated to the TypeScript extractor, template {fn(...)} calls (incl. the
multiline `{posts.map((post) => (` opening line), PascalCase component-tag
references. New astroResolver: Astro global + astro:* virtual modules as
framework-provided, component resolution with the #764 ambiguity rule,
src/pages/ file-based routes ([param]→:param, [...rest]→*rest, _-prefixed
and *.config.* excluded). SFC languages now preload the TS/JS grammars
their extractors delegate to (a pure-SFC file set previously had none
loaded). Also fixes a pre-existing Svelte/Vue script-block off-by-one that
reported every script symbol one line low.

Validated per the playbook: stalux (the issue's repro) 54/54 .astro files
indexed, getIconNode found at its exact line, 14/14 routes, 93.0% fair
cross-file coverage; AstroPaper 27/27 components, 13/13 routes (underscore
dirs correctly excluded), explore connects page→Card→Datetime through the
jsx-render synthesizer; node/edge counts stable across re-syncs.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/resolution/import-resolver.ts

@@ -24,6 +24,7 @@ const EXTENSION_RESOLUTION: Record<string, string[]> = {
   // `.svelte`/`.vue` file resolve to nothing, so barrel callers vanish (#629).
   svelte: ['.ts', '.js', '.svelte', '.tsx', '.jsx', '/index.ts', '/index.js', '/index.svelte'],
   vue: ['.ts', '.js', '.vue', '.tsx', '.jsx', '/index.ts', '/index.js', '/index.vue'],
+  astro: ['.ts', '.js', '.astro', '.tsx', '.jsx', '/index.ts', '/index.js', '/index.astro'],
   python: ['.py', '/__init__.py'],
   go: ['.go'],
   rust: ['.rs', '/mod.rs'],
@@ -582,9 +583,10 @@ export function extractImportMappings(
 
   if (language === 'typescript' || language === 'javascript' || language === 'tsx' || language === 'jsx') {
     mappings.push(...extractJSImports(content));
-  } else if (language === 'svelte' || language === 'vue') {
+  } else if (language === 'svelte' || language === 'vue' || language === 'astro') {
     // Svelte/Vue single-file components import via plain ES6 inside their
-    // `<script>` block. Without this, a `.svelte`/`.vue` consumer produces
+    // `<script>` block (Astro: the `---` frontmatter). Without this, a
+    // `.svelte`/`.vue`/`.astro` consumer produces
     // zero import mappings, so `resolveViaImport` can't run and a barrel
     // import (`import { Foo } from './lib'`) falls back to name-matching —
     // which silently fails whenever the re-export alias differs from the