edit-manifest — Predict-Verify Loop for Harness Edits

Implements two mechanisms from frontier harness research, named as in their source papers (both terms are the papers' own — verified 2026-06-17 against arXiv full text):

• Change Manifest (AHE, arXiv:2604.25850): each edit attaches a manifest entry declaring a falsifiable predicted impact (expected fixes + at-risk regressions); the next round intersects it with observed task-level deltas to produce a per-edit verdict. (Paper: "the Evolve Agent produces a change manifest…".)
• Validation Gate (SkillOpt, arXiv:2605.23904): a candidate is accepted only when its selection-split score is strictly greater than the current selection score (ties rejected); rejected edits are kept in an epoch-local buffer, not silently discarded. (Paper: "validation gate" / "held-out gate".)

FH's regression_guard.sh catches backward regressions. edit-manifest closes the forward loop: did the edit actually improve what it claimed to improve?