Speed up requests by opening staged databases in parallel and keeping them open

[CleanCut]

Give the staged database registry the per-path open lock and keep-warm that the refs registry already has.

• Move the registry onto WeakDbCache, holding the 64 most recently opened staged databases open after their last caller drops them.
• Evict the staged handles under a repository's workspaces directory before workspaces::clear removes it, so no warm handle holds RocksDB files open across the removal.
• Take OxenHub's test suite from 34.1s to 31.2s and liboxen's lib tests from 22.0s to 21.2s.

ENG-1243

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• crates/liboxen/src/core/db/weak_cache.rs

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📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Improved staged database handle management during opening, closing, and eviction for more reliable workspace operations.
  • Workspace clearing now releases staged database handles before deleting workspace data, preventing directory removal failures on Windows.
  • Added safeguards to preserve active database users and in-progress opens during closure.
  • Improved cleanup behavior when removing staged database entries, including related child entries.

Walkthrough

The change replaces the staged database’s manual weak-reference registry with WeakDbCache. It adds bounded warm-handle retention, coordinated opening and closure, prefix removal, and workspace cleanup before directory deletion.

Changes

Staged database handle lifecycle

Layer / File(s)	Summary
Cache close contract crates/liboxen/src/core/db/weak_cache.rs	WeakDbCache detects held slots, releases unheld warm handles, and reports whether closure occurred. Tests cover both close outcomes.
Staged database cache integration crates/liboxen/src/core/staged/staged_db_manager.rs	The staged database manager uses a 64-entry WeakDbCache for opening, closure retries, prefix removal, and shared handle management.
Workspace staged database cleanup crates/liboxen/src/repositories/workspaces.rs	clear removes staged database cache entries before deleting the workspace directory.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant staged_db_manager
  participant WeakDbCache
  participant RocksDB
  Caller->>staged_db_manager: open_staged_db(path)
  staged_db_manager->>WeakDbCache: get_or_open(path)
  WeakDbCache->>RocksDB: open on cache miss
  RocksDB-->>WeakDbCache: database handle
  WeakDbCache-->>Caller: shared handle
  Caller->>staged_db_manager: close_staged_db(path)
  staged_db_manager->>WeakDbCache: close(path)
  WeakDbCache-->>staged_db_manager: closed or still held

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Merge Risk: ⚪ Minimal · up to c937c

The cache now preserves in-progress opens during close races, and no merge-blocking regression was established.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description directly covers the WeakDbCache migration, staged database keep-warm behavior, workspace eviction, performance results, and issue reference.
Title check	✅ Passed	The title accurately summarizes the main change: parallel staged database opening and retaining databases open for faster requests.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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• Commit to this branch
• Create a new PR

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
In `@crates/liboxen/src/core/db/weak_cache.rs`:
- Around line 111-115: Update the slot-held check in the close/sweep logic
around get_or_open to preserve a slot with an in-progress opener: while holding
the slots write lock, treat Arc strong_count(slot) greater than one as held
before checking slot.handle. Retain the existing handle strong-count check for
other live handles.

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• crates/liboxen/src/core/db/weak_cache.rs
• crates/liboxen/src/core/staged/staged_db_manager.rs
• crates/liboxen/src/repositories/workspaces.rs

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