TypeSchema: collect referenced nested types into SnapshotProfileTypeSchema

[ryukzak]

Stacks on #158.

Makes snapshot.nested a closure of every nested type a consumer might reach when reading the snapshot, rather than just the profile's own nested array.

What the walk includes

• Profile-local nested types — seeded first so they take precedence by URL over inherited copies when constraints exist.
• Each nested type's fields — transitive nested-in-nested closure (e.g. Patient#contact → `Patient#contact.communication` if referenced).
• Profile's own constrained fields — anything the profile constrains directly.
• Base resource's fields — inherited types referenced by base fields matter to consumers that read inherited slots, since `flatProfile` only merges constraint-level fields.

Identities are preserved — same URLs, nested schemas shared by reference, not re-rooted to the profile.

Why now, what it unlocks

No immediate behavior change in the writer — the TS writer still imports nested types via `tsIndex.resolveType` and `tsModulePath`. But future code that reads methods/types from the snapshot directly (the larger plan) no longer needs to round-trip through the index for nested lookups. Snapshots become self-contained.

Tests

Two new tests in `test/unit/typeschema/utils.test.ts`:

• inherits nested types referenced by inherited fields from the base — verifies the base-resource walk picks up nested types the profile itself never references.
• prefers profile-local nested over inherited when URLs match — verifies precedence when a profile redefines a nested type with constraints.

Verification

• `bun run typecheck` clean
• `bunx biome check` clean (only pre-existing warnings)
• `bun test` 259/259 (was 257; +2 new)
• R4 + US Core examples regenerate byte-identical (writer doesn't yet consume `snapshot.nested` directly)

[ryukzak]

Closing — not required. The nested-collection groundwork wasn't unlocking anything concrete yet; we'll revisit if and when consumers actually need self-contained snapshots.