feat(certification): mutation survivors carry the edit that produced them

[IanFrelinger]

Follows #325. The gate reported a surviving mutant by id alone — mutate-int-literal-41 — which names an operator and a line and nothing else. That locates a survivor; it does not let you judge one.

The two causes of escape_rate > 0 need opposite responses. A weak witness wants more cases. An equivalent mutant — a rewrite that cannot change behaviour on any input — wants none, because no case can kill it and the candidate may be perfectly correct. Ledger S5 hit the second twice on semver-parse across two campaigns, and telling the cases apart meant decoding the recorded candidate by hand both times.

Change

Every AstMutation now carries a MutationSite (line, column, original text, mutated text, trimmed source line). BrickMutationEngine threads the survivors' sites onto MutationTestResult.Survivors, and CertificationGate spells them out in the rejection reason:

Mutation escape check failed: escape_rate=0.04, survivors=[mutate-int-literal-41]; mutate-int-literal-41 (line 41, col 13): 0 -> 2 in: if (version.Length > 0)

That line is all the adjudication needed for both S5 cases.

What does not change

• The verdict. Equivalence is undecidable, so EscapeRate > 0 still rejects and a human adjudicates. This is deliberately a reporting change only — a threshold that admits an equivalent mutant admits a real escape of the same rate.
• Mutant ids keep their exact {kind}-{line} format, so anything keying on them is untouched.
• The repair channel.RepairFeedback still renders survivors as locations only, gated by IncludeMutantLocations. Whether the proposer should see the mutant diff is a separate policy question (it is data about the candidate's own text, never the witness — but that decision is worth its own PR and measurement).

Tests

• AstMutationEngineTests.CollectMutations_CarriesTheEditThatProducedEachMutant — the S5 shape: asserts the int-literal mutant reports 0 -> 2 on if (version.Length > 0) and that the id still encodes the site's line.
• AstMutationEngineTests.MutationSurvivor_Describe_NamesLocationAndEdit.
• Existing gate-teeth, generation-safety, repair-feedback and damage-resolver dogfood suites: 33/33 locally; Nexo.Kernel.sln builds clean.

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