[sinkog]

What

Tests and one spec sentence only. Two cases in the merged tests, and a sentence in canonical-json.md, were meant to carry a u-escape and did not: the tool that wrote them had turned the escape into the character it stands for.

• plan_noop_exactness_test.go, "escaped text": it compared the same text twice, so it could not fail.
• m4a_gate_test.go, the "escapes" spelling in TestEquivalentSpellingsOfDesiredAndObservedGiveOnePlanDigest: identical to the plain spelling.
• canonical-json.md: the sentence read ("a" is "a") instead of ("<u-escape of a>" is "a").

The canonicalizer was never wrong, and the language-neutral vectors (canonical_json_vectors.json, plan_digest_vectors.json, written by a different route) do contain real escapes, so the property itself was covered. These two Go cases and the sentence were not what they claimed to be.

The change

• backslash is a hex-escaped constant ("\x5c") in m4a_helpers_test.go, so nothing between the file and the compiler can rewrite an escape built on it.
• Both cases now build their escape from it; the escapes row in the m4a test asserts that its inputs really contain a u-escape (a setup guard: it fails instead of silently testing identical text).
• The spec sentence is restored.

Evidence

• Mutant that stops decoding u-escapes in the canonicalizer: the two repaired cases fail; the previous versions of the same two cases passed under the same mutant (shown on the unmodified tests from devel). That is the point of the PR.
• Unmutated tree: the full suite passes; gofmt, vet, staticcheck clean; doc link check OK.

Not in this PR

No behaviour change in the module. No module.wasm change. No merge without explicit owner GO.

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