## Summary
Running more than one fusion process on a host (multiple dashboards/CLIs
across worktrees, all attaching `~/.fusion/fusion-central.db`) could
crash a `node` process at random — instantly, with no JS stack and
nothing in the logs. This happened 3 times in 3 days on one machine.
After this change those processes coexist without crashing.
The crash was an OS-level `SIGBUS` (`EXC_BAD_ACCESS`, `FS pagein error`
/ kernel `cluster_pagein past EOF`) inside SQLite's `walIndexReadHdr`.
In WAL mode every connection coordinates through a memory-mapped `-shm`
wal-index; on macOS/APFS, when one process resizes/rebuilds that file
during a checkpoint while another has it mmap'd, the reader faults on
the now-out-of-bounds page. A hardware memory fault can't be caught by
`node:sqlite` or JS, so the whole process dies.
The fix switches the central DB to `journal_mode = DELETE` (rollback
journal), which uses no `-shm` memory map and coordinates cross-process
access via POSIX byte-range locks instead — removing the faulting
surface entirely while keeping multi-process access. The existing
`busy_timeout` absorbs the writer serialization that DELETE mode trades
for WAL's reader/writer concurrency. Per-project DBs (`db.ts`) are
intentionally left on WAL: they're single-process-per-project and don't
hit this cross-process fault. SQLite migrates the existing WAL database
on first open (checkpoints `-wal` into the main file and removes
`-wal`/`-shm`), so there is no data loss.
## Test plan
- New regression tests in `central-db.test.ts` assert the central DB
reports `journal_mode = delete` (not `wal`) and that **no `-shm`
wal-index file is ever created** even after write traffic — i.e. the
exact faulted surface is gone.
- All 6 central-DB suites pass (221 tests); `@fusion/core` typechecks
clean.
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## Summary by CodeRabbit
* **Bug Fixes**
* Improved stability when multiple dashboards or CLIs run on the same
machine.
* Switched the local database to a safer journaling mode to reduce rare
crash issues on macOS/APFS.
* Prevented creation of extra database side files during normal
operation, while keeping data durability and lock-based coordination in
place.
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