fix: Lazy grammar loading and quantized embeddings to prevent V8 WASM OOM

Fixes #54 — `codegraph init -i` crashes with "Fatal process out of memory: Zone"
on large codebases because all 16 tree-sitter WASM grammar modules were compiled
upfront by V8, exhausting the WASM Zone allocator.

Changes:
- initGrammars() now only initializes the tree-sitter WASM runtime (Parser.init()),
  no longer eagerly loads all grammar files
- New loadGrammarsForLanguages() loads only grammars for languages actually present
  in the project (e.g. a Dart project loads ~2-3 grammars instead of 16)
- Orchestrator detects needed languages after file scan, before parsing begins
- Embedding pipeline now uses quantized model (~67MB vs ~270MB) to further reduce
  WASM memory pressure when embeddings are enabled

__tests__/pr19-improvements.test.ts

@@ -25,10 +25,12 @@ import {
   clearParserCache,
   getUnavailableGrammarErrors,
   initGrammars,
+  loadAllGrammars,
 } from '../src/extraction/grammars';
 
 beforeAll(async () => {
   await initGrammars();
+  await loadAllGrammars();
 });
 
 // Create a temporary directory for each test