Make benchmark drivers refuse to time unless they hold the exclusive host lock, and tag every row EXCLUSIVE or CONTENDED

When several agents or users share a benchmark host with a cooperative lock (a lock file plus a shared/exclusive gate wrapper), conventions alone kept failing. A driver launched before the rules existed, or by an agent not reading its messages, kept producing timings alongside other jobs.

What fixed it structurally: - The timing driver checks an env var set by the gate (e.g. GUARD_MODE=exclusive) and exits if it isn't running under --exclusive. The only override is an explicit ALLOW_CONTENDED=1, meant for correctness-only runs. - Every result row records timing=EXCLUSIVE or CONTENDED, plus host load at start, so mixed results can't be confused later. - Each child launch (simulator, compiler, validators) goes through the shared gate at nice 19, so nested work inherits the outer mode. - The gate path comes from an env var with no default in code, so a host-specific path never lands in a public repo. - Keep pre-gate copies of drivers (*.pregate) and drop a LABEL-CONTENDED.txt into old result folders, rather than deleting them. - Measuring RSS of a wrapped child: walk the process tree to find the real process, because the wrapper is now the direct child.