Add VexFS filesystem support for SQLite and PostgreSQL

[vibeinging]

What changed

• add VexFS as a database-managed filesystem for VexDB Lite
• provide unified vexdb fs / vexfs commands, local shell workflows, archive import/export, version history, snapshots and restore
• add SQLite local storage and PostgreSQL shared-workspace storage; DuckDB is intentionally out of scope
• add macOS NFS/FSKit integration and Linux FUSE integration, including package and platform checks
• add PostgreSQL staged block publishing, background batching, backup/recovery, cross-gateway visibility and indexed grep evaluation
• harden PostgreSQL reconnect behavior with bounded asynchronous libpq I/O and cache resynchronization after lost notifications
• harden macOS gateway cleanup against PID reuse by validating the process start identity, mount source and filesystem identity before signaling

Why

VexDB Lite should expose database-owned file management that agents can use through normal filesystem commands while retaining database permissions, history, backup and remote workspace sharing.

The final recovery work closes three mount-safety gaps found during review: a lost PostgreSQL notification could leave stale metadata cached, an established but non-responsive TCP connection could block a mount operation indefinitely, and a reused macOS PID could target the wrong process during gateway cleanup.

User impact

• local SQLite workspaces can be mounted and used from Bash-compatible tools
• PostgreSQL workspaces can be shared across gateways and recover after connection or database restarts
• file versions and snapshots remain managed by the database
• network failure returns within a bounded time instead of hanging the filesystem
• stale gateway records cannot kill an unrelated reused PID

Validation

• SQLite spec: 30 passed, 0 failed
• PostgreSQL libpq runtime: 134 checks passed
• PostgreSQL network cut and blackhole recovery: 31 checks passed; established and new-connection blackholes fail in about 5 seconds; oom_kill=0
• PostgreSQL strict crash recovery: 21 checks passed; oom_kill=0
• macOS Release build plus runtime, static and platform registry smoke checks passed
• Linux AArch64 full Release build passed with a 1 GiB memory limit and one build job, covering the SQLite loadable/static targets, VexFS runtime, CLI, FUSE helper and smoke binaries
• git diff --check passed

Scope boundary

DuckDB filesystem adapter work is not included in this PR.