fix(rpiv-pi,rpiv-site): review one-liner batch — base=HEAD clause, pinned invariants, resynced prose (review I4/Q5/Q10/Q12/Q15/Q18/Q21)

- validate Step 2.6 defines <base> for the pre-commit context: when the
  implementation is uncommitted (the workflow's normal case — validate runs
  before commit) <base> is HEAD, so the byte-identical downgrade proof is
  deterministic instead of improvised (I4 — the review's "unresolvable"
  claim was wrong, git diff --quiet HEAD -- <file> is exactly the proof,
  but the skill never said so).
- foldScopeVerdict pins the partition's input assumption: only "??" counts
  as untracked; a STAGED new file deliberately classifies as tracked excess
  — the conservative direction, adjudicated in place, never quarantined (Q5).
- goal-baseline's --scope comment corrected: the flag is absent only when
  the floor never ran — writeScopeVerdict publishes on a clean pass too (Q10).
- plan/synthesize achievability-rule twins harmonized (eslint example +
  evidence wording now identical) and both carry an explicit mirror note (Q12).
- site mirror header prose resynced to 32 runtime stages, honoring its own
  keep-in-sync contract (Q15).
- ship routing test: the tiered "untracked-only"/"excess" verdicts STOP at
  ship's pass-only gate — the stop-on-fail contract is now asserted, not
  just commented (Q18).
- built-ins barrel re-exports the ScopeVerdict type alongside its values (Q21).

Assessed but deliberately NOT applied: Q20 (ship-side --scope threading
test — ship shares the identical VALIDATE_GOAL_PROMPT function object;
zero marginal coverage) and Q12's shared-include form (skills have no
markdown include mechanism; harmonize-plus-mirror-note is the available fix).