[P3][kernel] Throw libredb error when close() is called during transact()

[cevheri]

Summary

Calling close() while inside transact() can surface raw Node EBADF from the filesystem instead of a structured libredb: error.

Audit ID: Section 4 Low — Wave 3
Location: src/core.ts:517-521

Fix

• Detect inTransaction on close() → throw libredb: cannot close during an open transaction before touching the log.
• Or defer close until transaction completes — prefer explicit throw for sync API clarity.

Tests

• transact(() => { db.close(); }) → libredb error, not EBADF.

Acceptance criteria

• Stable libredb error.
• bun run gate green.
• Changeset required.

[cevheri]

Implemented and shipped in PR #43 (merge commit 69f7161), released as v0.2.0. The PR description's comma-separated Closes list only auto-closed the first reference — closing manually. For #39/#28-style partials: the shipped scope is done; the real-browser verification half of #39 continues under #10, and the typed-error contract (#28) shipped as LibreDbError with stable codes across kernel, adapters, lenses, and CLI.