fix: bound Cursor blob-store memory

[Ingwannu]

Summary

• cap Cursor blobs at 16 MiB each, 64 MiB total, and the existing 4,096 entries
• pin every root/step/turn blob advertised by an active request until success, failure, or generator cancellation
• evict only expired or LRU unpinned blobs; fail preparation when pinned data leaves no safe capacity
• return a protobuf error for rejected server setBlobArgs instead of acknowledging a store that did not happen
• store external root candidates only after replay pruning

Part of #820. This is a focused request-lifetime lease implementation rather than #829 app-wide memory infrastructure.

Verification

• bun test tests/cursor-*.test.ts (398 pass)
• bun run typecheck
• bun run privacy:scan

Compatibility

Content-addressed IDs and get/set protobuf shapes remain unchanged. A single blob above 16 MiB or a request whose unique pinned blobs cannot fit 64 MiB now fails explicitly rather than evicting an in-flight reference or retaining unbounded data.

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