Filesystem storage now enforces single-writer, many-reader semantics.
A second writer on the same data directory throws at init time with the
holder's PID, hostname, and heartbeat — replacing the previous silent
stale-reads failure mode.
- New: `Brainy.openReadOnly()` — coexists with a live writer, every
mutation throws clearly.
- New: writer lock at `<rootDir>/locks/_writer.lock` with 10s heartbeat
and stale-detection (PID liveness + heartbeat freshness).
- New: cross-process flush-request RPC (filesystem-based, no signals)
so inspectors can force fresh state on demand.
- New: `brain.stats()`, `brain.explain(findParams)`, `brain.health()`
for operator-facing introspection.
- New: `brainy inspect` CLI with 13 subcommands (stats, find, get,
relations, explain, health, sample, fields, dump, watch, backup,
repair, diff), all read-only by default.
- Same-PID re-opens allowed with a warning (preserves test "simulate
restart" patterns).
- Storage instances passed directly via `storage: new MemoryStorage()`
are now honoured instead of silently falling through to the
filesystem auto-detect path.
Brainy + Cortex compose under this model — the lock covers both because
they share `rootDir`, Cortex segments are immutable mmap files, and
MANIFEST updates use atomic-rename.
