Shared memory header versioning and compatibility checks

[toloco]

Priority: High impact / small effort

Why

Prevent corruption or undefined behavior when layout changes.

Acceptance Criteria

• Header contains:
  • Magic number
  • Version
  • Header size
  • Endianness or architecture flag
• Opening incompatible shm fails with a clear error

[toloco]

Resolved. Shared memory header versioning is fully implemented: src/shm/layout.rs defines magic bytes (FCACHE01), VERSION=3, and validates on open (src/shm/region.rs:165-172). Incompatible parameters trigger automatic recreation. Landed across PRs #19–#22.