The C side of #255 ships in the
transpiled SQLite 3.53.4 sources: opt-in, off by default, process-wide,
frozen at the process's first lock attempt, with a latched POSIX fallback
on kernels that reject F_OFD_*. This adds the Go-facing half: OFDLocking
switches the mode (overriding MODERNC_SQLITE_OFD_LOCK) and
OFDLockingEnabled reports it, with ErrOFDLockingTooLate and
ErrOFDLockingUnavailable mapping the gate's refusals. The scenario tests
from merge request !136 now run in a re-executed child process with the
mode really on (and the both-modes invariants also in the parent's
inherited mode), TestOFDLockingSetter covers the Go call path end to end
against /proc/locks, and the other platforms assert the switch reports
itself unavailable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qsot8zfLpqv4jHfpRgEq94

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qsot8zfLpqv4jHfpRgEq94