docs(03-04): approve replay divergence diagnostic contract

- Record first-divergence thrown behavior and locked fields
- Preserve the public Promise<boolean> replay interface

.planning/phases/03-data-completion/03-REPLAY-DIAGNOSTICS.md

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+# Phase 3 Plan 04: Replay Diagnostics
+
+## Approval
+
+`approve-locked-contract` was received for the first-divergence verifier
+diagnostic. This record is implementation-local to the Node verifier and does
+not define a new public replay error mechanism.
+
+## Locked failure behavior
+
+The verifier stops immediately at the first replay divergence and throws an
+implementation-local error. The stop conditions are:
+
+- an unknown command code;
+- a command returning `false`;
+- a command throwing; or
+- a final hero or map snapshot mismatch after the sandbox has ended normally.
+
+The existing replay command contract remains `execute(step): Promise<boolean>`.
+The replay command interface is not changed to carry diagnostics. The verifier
+does not add a user-facing error class, a public diagnostic-return object,
+logging-only failure handling, or continued execution after a divergence.
+
+## Required diagnostic fields
+
+Every thrown verifier-local diagnostic reports exactly the locked diagnostic
+information needed for triage:
+
+| Field | Meaning |
+| --- | --- |
+| `index` | The first divergent command index. For an end-only snapshot mismatch, this is the route index at which final verification reports the mismatch. |
+| `code` | The top-level stable replay enum code from Plan 03. An unknown code is reported unchanged rather than being inferred or renumbered. |
+| `params` | The original route parameters for the reported command, before diagnostic formatting. |
+| `reason` | A readable explanation of the first divergence. |
+
+The verifier may display `params` using a deterministic safe representation for
+diagnostic output. This representation must preserve parameter order and
+primitive meaning, handle `bigint` without lossy JSON conversion, and avoid
+executing or invoking user-provided behavior. It is display formatting only; it
+does not alter the replay route or the public `Promise<boolean>` contract.
+
+## Scope boundary
+
+This record approves the local thrown diagnostic and its four fields only. It
+does not establish a public error class, a public diagnostic interface, a
+diagnostic-return API, new replay command codes, or additional output fields.