feat(scanners): the subtraction axis can now remove what it proposes (SUB-WRITE)

`optimize --subtract` has only ever proposed. `--apply` executes the blocks the
operator picks, behind a backup whose coverage is verified and a scope gate the
engine enforces rather than describes.

The open design decision from plan §C6 was settled by two measurements, not by
taste. It is NOT a fix-engine action: the subtraction axis appears nowhere in
scan-orchestrator or optimization-lens-scanner, so verifyFixes' re-scan would
mark every removal `verified` whether or not it happened -- a success-shaped
no-op, the same shape that made restoreBackup silently do nothing. It is NOT a
plan/implement step either: that pipeline needs a finding code, and OPT declares
exactly one, for the deterministic check.

The approval artifact is written by main context, not by the lens agent. That is
where the operator's decision actually happens, and it keeps the feature off the
still-unmeasured agent write surface (M-BUG-18 lists optimize as open).

Three properties are load-bearing, and each was seen red against its own defect:
removals validate against the ORIGINAL content and apply in descending line
order; the range check is not redundant with the text check (`line: 0` makes
`slice(-1, 0)` empty, so an empty text MATCHES and `splice(-1, 1)` deletes the
file's last line); and createBackup skips a nonexistent path while still
returning an id, so manifest coverage is asserted before a byte changes.

Two guards were green on their own defect and were fixed after measuring:
`/\b80\s*%\b/` never matches "80% of the file" -- `%` is a non-word character, so
the trailing `\b` demands a word character next. And the caller-arm sweep passed
vacuously against HEAD, iterating an empty list; only the added non-emptiness
assertion caught it.

The floor is repeated, not moved: floor-exclusion still vetoes before anything is
proposed, and the engine refuses a load-bearing block again so a hand-built
approval cannot route around it. `mv` to `_archive/` is a file-level rule and
does not apply to a block excision -- the timestamped backup is the recovery
artifact, and a second copy with no restorer would be worse than none.

strongestGate moves into write-scope.mjs so the gate ordering has one owner.

Dogfooded DRY-RUN against the real ~/.claude/CLAUDE.md: 29 candidates, gate
refused all 29 with exit 0 until the scope was approved, then 29/29 spans
validated with nothing written. ~789 tokens, ~18% of the file -- corroborating
the #40 fasit's ~850, and well short of what a deletion feature is tempted to
promise.

Suite 1625 -> 1659/0. Frozen v5.0.0 and default-output baselines untouched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017A6vrtPKsVuM4DJ27p7jzw

CLAUDE.md

@@ -17,7 +17,7 @@ Per-command flags, patterns, and feature lists live in `README.md` and `/config-
 | `/config-audit tokens` | Prompt-cache-aware token hotspots, each tagged with its load pattern; cache-aware |
 | `/config-audit manifest` | Ranked table of every token source + always-loaded subtotal |
 | `/config-audit feature-gap` | Context-aware feature recommendations grouped by impact |
-| `/config-audit optimize` | Mechanism-fit lens (procedure→skill, lifecycle→hook, path→rule, never→permission). Agent-driven, **not byte-stable**. `--subtract` adds the subtraction axis (what no longer earns its always-loaded rent, `BP-SUB-001`) — opt-in, proposes only |
+| `/config-audit optimize` | Mechanism-fit lens (procedure→skill, lifecycle→hook, path→rule, never→permission). Agent-driven, **not byte-stable**. `--subtract` adds the subtraction axis (what no longer earns its always-loaded rent, `BP-SUB-001`) — opt-in, proposes only; `--subtract --apply` executes the removals the operator picks |
 | `/config-audit fix` | Auto-fix deterministic issues with backup + verification |
 | `/config-audit rollback` | Restore configuration from backup |
 | `/config-audit plan` | Create action plan from findings |
@@ -97,6 +97,8 @@ Coding style: scanners are zero-dependency Node ESM; new findings use the `CA-{S
 
 **Subtraction floor (invariant).** `optimize --subtract` is the only lens that proposes removing config, so `scanners/lib/floor-exclusion.mjs` runs as a deterministic pre-step *before* the judge — a load-bearing block is never a candidate, and that guarantee must not be moved into the agent prompt. Two rules follow from it: (1) **staleness is not a deletion signal** — an outdated version pin inside a floor block is a `drift`/`CA-CML` dead-reference concern; (2) **tier 2 ≠ tier 3** — a compensatory block that keeps earning its place returns, and reporting it as dead weight is wrong even when the label matches. Norwegian keywords need the Unicode boundaries in `subtraction-prefilter.mjs`; JS `\b` is ASCII-only, so `/\bunngå\b/` silently never matches.
 
+**Subtraction write path (invariant).** `--apply` routes through `scanners/lib/subtraction-write.mjs`, never through `fix-engine` or the `plan`/`implement` pipeline, and both exclusions are **measured**: the subtraction axis is absent from the orchestrated envelope, so `verifyFixes`' re-scan would mark every removal `verified` whether or not it happened (a success-shaped no-op), and the findings pipeline needs a finding code — which names a deterministic check, not a prose judgement. Three properties are load-bearing and each has a guard seen red against its own defect: removals are validated against the ORIGINAL content and applied in **descending** line order (an ascending pass shifts later spans out from under themselves); the **range** check is not redundant with the text check (`line: 0` makes `slice(-1, 0)` empty, so an empty `text` matches and `splice(-1, 1)` deletes the file's LAST line); and `createBackup` skips a nonexistent path while still returning an id, so coverage of every file about to be written is **asserted from the manifest** before a byte changes. The floor is *repeated* here, not moved: `floor-exclusion` still vetoes before anything is proposed, and the engine refuses a load-bearing block again so a hand-built approval cannot route around it. The archive rule (`mv` to `_archive/`) is file-level and does not apply to a block excision — the timestamped backup is the recovery artifact, and inventing a second copy with no restorer behind it would be worse than none.
+
 ## Testing
 
 ```bash