fix: App Router route-group and slot collision

[JaredStowell]

Fix App Router collision handling when multiple filesystem entries resolve to the same visible URL.

This changes route discovery and validation to:

• reject duplicate normalized route patterns after route-group elision
• treat /foo and /foo/ as the same route during validation
• reject grouped slot sub-pages that collapse to the same URL within the same slot
• reject slot sub-pages that collide with route.ts handlers
• preserve shadowing semantics for same-named slots at different layout depths
• merge matching slot sub-pages from different slots onto the same synthesized route

Details

In app-router.ts:

• track slot ownership with internal ownerDir metadata
• key synthetic slot merges by (slot name, owner dir) instead of slot name alone
• normalize visible slot subpaths before duplicate detection
• throw on page-vs-route-handler collisions
• stop generating slash-suffixed synthetic patterns for empty/group-only subpaths

In route-validation.ts:

• canonicalize trailing slashes before duplicate checks and dynamic-route validation

Tests

Added regressions for:

• duplicate normalized patterns
• slash-equivalent patterns
• route-group duplicates like (a)/about vs (b)/about
• grouped slot sub-page duplicates within one slot
• grouped slot sub-pages from different slots merging correctly
• slot sub-pages colliding with route.ts
• parent slot sub-pages not overwriting a shadowing child slot of the same name

Verification

Passed:

• pnpm test tests/route-sorting.test.ts
• pnpm test tests/routing.test.ts
• pnpm test tests/app-router.test.ts
• pnpm run fmt
• pnpm run typecheck

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commit: 0ee2420