State in C-132 that a mut-param write made before an err is visible to the caller

[O6lvl4]

What

Carries the ruling on almide/almide#2917 (#1871 option (B), "write visible") into the judge.

• C-132: the #1576 part said the err arm carries no buffer and that a ! site propagates before any write-back, so the caller keeps its pre-call value. That is withdrawn. Its "Boundary … stays walled" sentence is dropped. It is replaced by an AMENDED paragraph:

  • A write the callee makes to its mut param before it errs is visible to the caller, as with native &mut.
  • Every raising exit carries the buffer it holds at that point: an explicit err(..), a guard, every x! / x?.
  • The call site writes back on both arms.
  • An unpropagated call (??, match, bound Result, let _ =) writes back, then yields ok/err.
  • A ! call writes back, then propagates.

  The exclusion of a declared-Result effect fn whose err type is not String is kept.

• C-226: it no longer describes the value-returning mut-param effect fn as "the documented C-132 exclusion". It points at C-132's ok and err arms.

• New fixture spec/wasm_cross/mut_param_err_write_visible.almd:

  • exits after a write: explicit err, guard, propagated !;
  • consumers: ??, match, bound Result, let _ =, and a ! forwarded one level up;
  • buffers: List, String, record.

• The header comment of mut_param_effect_can_err.almd no longer states the old order.

The reference evaluator already copies mut finals out on every exit (call_fn_muts), so the ref needs no change. On the new fixture, als-ref run and native almide 0.64.0 print identical stdout, exit 0. The wasm legs wall on it today; almide/almide#2917 implements it.

ALS-M13's prose is general ("written back at every call position") and does not contradict this, so it is left unamended. Amending it would need a fresh review stamp in proofs/als-validation.toml.

Author / verifier record

role	who (human handle, or agent + session)	independent of the author?
authored	Claude Code agent (Opus 5.5), for O6lvl4	—
verified	same agent: ref evaluator + native almide 0.64.0 run, local gates	no

Ratchets loosened in this PR (ceiling up / floor down): none.