New and incremental core databases do not reclaim deleted pages during their lifecycle

Description

Scoped follow-up to #31526: new core databases use auto_vacuum=NONE, and opening an already-INCREMENTAL database or deleting an event aggregate does not reclaim its free pages.

Expected: enable incremental reclamation for new databases and make bounded, best-effort progress on compatible databases after initialization and aggregate deletion. Preserve retained records and existing NONE/FULL modes; do not perform a full startup VACUUM.

Legacy database conversion and live-event retention remain outside this issue, tracked by #31526 and #33356. Draft #47589 implements this limited fix; this issue records its acceptance scope without closing those broader reports.

Plugins

None in the isolated reproduction.

OpenCode version

Source dev at 337fd144d2ba144743368f78d9579a99cce175bd.

Steps to reproduce

1. Initialize a disposable database through Database.layerFromPath; inspect PRAGMA auto_vacuum (currently 0).
2. In a separate disposable fixture, enable INCREMENTAL offline, insert and delete a 4-MiB payload, retaining one small record.
3. Reopen through the same layer: the freelist does not decrease. Aggregate deletion likewise has no reclamation pass.

Reproduction tests are in #47589: from packages/core, run bun test test/database-storage.test.ts. The new-file and reclamation assertions fail against the original implementation and pass with the proposed patch. No real session data is needed.

Operating System

macOS, Bun 1.3.14; additional production-layer smoke test under Node 24.15.0.

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