Add priority-aware host reclamation transport

[kylebernhardy]

Adds the first slice-6 lifecycle unit for the experimental Harper-backed Tantivy directory in
Implement Harper-backed Tantivy Directory mapping and atomic publication.
This remains test-only and does not expose the Harper storage backend in the release artifact.

Architecture

• Adds one low-priority cleanup admission class to the existing per-index host transport; it shares
  the foreground callback channel while allowing at most one cleanup operation in flight.
• Preserves foreground operation and byte headroom independently. Cleanup fails fast when current
  capacity is unavailable, while permanent configuration/request-size errors are classified before
  transient occupancy.
• Constructs the cleanup KvDirectory from the foreground store, namespace, and ReclaimBudget in
  one operation. This preserves the exact storage identity and reader-pin state and prevents request
  bounds from being paired with a different namespace.
• Keeps a timed-out response charged until its dispatched callback completes or the transport closes;
  admission and response timeouts remain distinct.
• Treats N-API Closing as normal shutdown and fails the transport closed for other callback-queue
  inconsistencies.

For the human reviewer

Two deliberate contracts remain for the later production lifecycle unit:

1. One shared FIFO host callback channel is used initially. The design requires a measured foreground
   p99 gate and moves cleanup to a separate callback channel if bounded work cannot remain within it.
2. max_operations is sized as configured foreground concurrency plus one cleanup slot. The transport
   enforces one cleanup operation; demand beyond the configured foreground concurrency waits normally.

The dedicated cleanup task will bind and own its ReclaimBudget, expose health, and prevent use of
non-reclamation directory methods. Those concerns are intentionally outside this test-only transport
slice and must be complete before Harper enables the backend.

Verification

• npm run check: formatting and TypeScript checks passed; Clippy passed with warnings denied; 108
  Rust tests, 42 active Node/native tests, and the packed consumer artifact test passed. Three existing
  opt-in rocksdb-js lease tests remained skipped because no feature-enabled checkout was configured.
• npm run benchmark:smoke: passed against the unchanged public mmap backend (1,000 documents; warm
  approximate p99 0.229 ms in this run).
• The host-transport integration test proves same-identity reader-pin visibility, exact request-bound
  sizing, physical deletion of both retired payloads, close wake-up, worker teardown, and conservative
  timeout accounting.
• Independent Claude and direct Gemini reviews found and drove corrections to permanent/transient
  error classification, request-bound enforcement, namespace binding, shutdown taxonomy, and test
  overclaims. Both final convergence reviews found no material remaining defect; one Gemini diff-only
  clamp finding was rejected because the shared helper already applies the same clamp.

Complexity: complicated

Comment generated by kAIle (GPT-5)

_{Review-Coverage: authored=unknown; ran=none; rounds=1 @ f161f5e}

_{Human-Review-Need: 4 @ f161f5e}