feat: per-entry hooks to filter, reshape, and rewrite snapshots

[antfubot]

Closes #38

Adds three per-entry hooks to ApiSnapshotOptions, at increasing depth of intervention:

• entryFilter(ctx) — skip an entry point entirely (ctx: { packageName, entryName }; return false to skip). Solves the UnoCSS case of dropping low-signal data entries like ./theme / ./colors while keeping the rest of the package guarded.
• transformEntries(entries, ctx) — modify the internal structural representation (the extracted Entry[] of { name, kind, text } per surface) before it is serialized: mutate in place or return a replacement array. Enables e.g. collapsing a known large object export to an opaque declaration while everything else stays guarded.
• transformSnapshot(ctx) — rewrite the final snapshot string (header excluded) before it is written or compared; return the new string or null/undefined to keep it.

Because the hooks live on ApiSnapshotOptions and are bound through a single shared createEntryHooks helper, they behave identically across every integration: snapshotApiPerEntry / describePackagesApiSnapshots (Vitest), generateApiSnapshot, snapshotPackage / snapshotFiles, and the rolldown plugin. In the Vitest integration, filtered entries are skipped at test-registration time (no dangling it blocks), and transformed content flows through the breaking-change update guard, so the guard compares what will actually be written.

Also, as noted in the issue, typeWidening and categorizedExports were not forwarded through the Vitest SnapshotApiOptions — they now are, and the Entry / EntryKind / hook-context types are exported from both tsnapi and tsnapi/vitest.

Covered by new core tests (test/hooks.test.ts) and a hooked fixture in the Vitest integration test with committed snapshots.

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This PR was created with the help of an agent.