feat(graph): add Next.js App Router route resolver

[abhinav-phi]

Resolves #95.

What

A bounded Next.js resolver following the express.ts/flask.ts pattern. It turns App Router route modules into route nodes and connects exported HTTP handler functions:

• Recognizes app/**/route.ts and route.js, including src/app roots
• One stable route node per exported handler for GET, POST, PUT, PATCH, DELETE, OPTIONS, HEAD — declared as export async function GET() or export const GET = async () => ...
• Route path derived from the route file's directory: app/api/users/route.ts → /api/users
• Dynamic segment text ([id]) and catch-alls ([...slug]) preserved verbatim; route groups ((marketing)) excluded from paths the way Next itself resolves them
• Only explicitly exported HTTP-handler names create route nodes; a same-file helper() export is ignored
• Detection keys on the next dependency in package.json or the presence of route files in the tree (positive and negative cases tested)
• Same-file handlers resolve only when unambiguous; missing or duplicate handlers stay unresolved

Out of scope, per the issue: Pages Router, route groups' URL composition nuances, parallel/intercepting routes, rewrites, middleware, server components, and any change to the TypeScript extractor or graph core. No identity, reconciliation, schema, or drift-semantics changes.

Tests

• resolver-nextjs.test.ts (11): detection positive/negative (manifest and file-based), path derivation for both roots plus route groups/dynamic/catch-all segments and rejection of non-App-Router paths, function and arrow-function exports, JS route files, non-route files ignored, resolution and ambiguity, registry registration.
• resolver-nextjs-integration.test.ts (1): real rebuildGraph over a Next.js fixture — route nodes persist with the stable nextjs-route identity and both handlers resolve through framework edges.

npm run typecheck, npm run build pass; resolver suites green locally (the pre-existing Windows symlink/WAL failures in test/graph-integration.test.ts reproduce identically on clean main and are unrelated).