P13-S27: grant one narrow, explicit exception to "no execution work"

Authorises a bounded mechanical probe of M7's experiment on a disposable
branch, for evidence only, before review round 11. Granted before use
because the prohibition is otherwise absolute.

Scope is the part of M7 runnable against the tree as it stands: whether a
base-free Bundle -> text -> Bundle round trip reproduces the original
image() bytes. That is M7's load-bearing assumption after round 10 and the
thing four paper designs never established. Result and complete diff
recorded, branch discarded, nothing merged.

Explicitly not authorised: any S27 implementation (no BundleCapabilities,
no capabilities(), no pin 3a validation, no CanonicalBaseRequiresRebuild,
none of tests 1 through 10b); any staging or commit on main beyond this
contract's own rows; any pin, test, gate, mutation or touch-table change
arising from the probe without its own review round. The probe produces
evidence, not amendments.

Recorded as evidence in its own right: M7 as written cannot be executed
until S27 has landed. BundleCapabilities and
CURRENT_REDUCTION_ALGORITHM_VERSION do not exist in the tree -- they are
S27's own deliverables -- and M7 step 1 requires a base committed under the
real authority. M7 is a mutation of this rung's implementation, and
mutations run after the rung, not before it. The probe therefore tests the
round-trip machinery M7 depends on, not M7.

The probe is base-free, so it removes no refusals: both
project_text_document and serialize_document gate on
canonical_base.is_some(). It touches none of pin 3b's guards, cannot leave
one unrestored, and leaves the live no-canonical-base constraint intact.

Still NOT RATIFIED, NOT DISPATCHABLE. Pins unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ps1szk2mSfgp4Cz21eVH9x

spec/CONTRACT_P13S27_REDUCTION_AUTHORITY.md

@@ -14,7 +14,52 @@ nothing.
 
 **The pins are therefore NOT frozen.** They are open to **the current round's**
 findings. Freezing follows ratification; it does not precede it, and it does not
-survive a withdrawal. **No execution work may begin** — not implementation, not staging,
+survive a withdrawal.
+
+### One narrow, explicit exception to "no execution work" — granted 2026-08-08
+
+**Granted before use, because the prohibition below is otherwise absolute.**
+Authorised: **a bounded mechanical probe of M7's experiment**, on a **disposable
+branch or worktree**, for the sole purpose of producing evidence for the next
+review round.
+
+**Scope — what is authorised:**
+
+- **Only M7's mechanics**, and only the part runnable against the tree as it
+  stands: **whether a base-free `Bundle` → text → `Bundle` round trip reproduces
+  the original `image()` bytes.** That is M7's load-bearing assumption after
+  round 10, and it is the thing four paper designs never established.
+- The exact result and the **complete diff** are recorded, then the branch is
+  **discarded**. Nothing merges.
+
+**Scope — what is NOT authorised, and is not a judgement call:**
+
+- **No S27 implementation.** No `BundleCapabilities`, no `capabilities()`, no pin
+  3a validation, no `CanonicalBaseRequiresRebuild`, none of tests 1–10b.
+- **No staging or commit on `main`** beyond this contract's own amendment rows.
+- **No pin, test, gate, mutation or touch-table change** arising from the probe
+  without its own review round. **The probe produces evidence, not amendments.**
+- **No canonical base anywhere** — the live constraint (§1.2) is untouched, which
+  is exactly why the probe is base-free.
+
+**Why the probe cannot cover M7 in full, which is itself evidence for round 11.**
+`BundleCapabilities` and `CURRENT_REDUCTION_ALGORITHM_VERSION` **do not exist in
+the tree** — they are S27's own deliverables. M7 step 1 requires a base committed
+**under the real authority**, so **M7 as written cannot be executed until S27 has
+landed.** M7 is a mutation *of this rung's own implementation*, and mutations run
+after the rung, not before it. **The probe therefore tests the round-trip
+machinery M7 depends on, not M7.**
+
+**A base-free probe removes no refusals at all** — both `project_text_document`
+(`project.rs:580`) and `serialize_document` (`serialize.rs:151`) gate on
+`canonical_base.is_some()`. So the probe touches none of pin 3b's guards and
+cannot leave one unrestored.
+
+**What the probe decides.** If base-free round-tripping is **not**
+byte-identical, M7's whole-image comparison is unsound **regardless of bases**,
+and round 10's fix is wrong too. If it **is**, the comparison design survives its
+first contact with the code and only the base leg remains unverified — pending
+S27. **No execution work may begin** — not implementation, not staging,
 not partial work against "the settled pins."
 
 **History — the running tally, which has now gone stale three times and been
@@ -1602,6 +1647,11 @@ to 1 belongs to S16.
 
 **The executing agent MUST NOT commit.** Leave the work staged.
 
+**No execution work may begin at all** until this contract is ratified — **with the
+single exception granted in the status block above**, which authorises a
+base-free, discard-only mechanical probe of M7's round-trip machinery on a
+disposable branch, and nothing else.
+
 ---
 
 ## §7. Report requirements