plan: mut.1 — AST extension + Form A surface for mut/var/assign
Six tasks: AST + serde (Task 1), substantive walker arms across the ~25 exhaustive Term match sites (Task 2), the two dispatch stubs in check_term and lower_term that produce CheckError::Internal / CodegenError::Internal per the spec's out-of-iteration boundary (Task 3), Form A parser + printer (Task 4), schema-drift + coverage + spec-drift extensions + form_a.md + DESIGN.md amendments (Task 5), examples/mut.ail fixture with round-trip + corpus-coverage gate (Task 6). Three Boss decisions made explicit in the plan: walker-arm policy (substantive everywhere except the two dispatch entry points), empty-vars canonical-bytes pin, parse rejection of (mut) without body. Plan-recon flagged all three; the plan resolves them rather than deferring.
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