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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[lidge-jun]

Maintainer triage status: this is valuable work. It bounds Cursor blobs while using scoped pins to protect active requests, has strong test coverage, and defines saturation behavior explicitly. It currently conflicts with dev, so a rebase is needed. It also shares the Cursor transport/docs surface with #844; please land the two in a defined order (or consolidate them) to avoid overlapping integration work. An independent, non-author review is required before landing.