tooling: re-runnable measurement for the stall guard's p+s headroom

The stall guard's budget depends on p+s (job prep plus how far into the
guarded step output froze). check-stall-guard-budget judges C<T and
T-C>=W, all statically readable; p+s is bounded by the healthy run
length, which no static sweep can read, so it was carried in prose and
had already gone stale.

This is the re-derivation, so the number can rot loudly instead of
silently. It imports the gate's own sweep rather than keeping a second
list of guarded step names.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CPrUz21stTFhJRUirdc4yw

Author: claude