Seven tasks: three new CheckError variants + code() + ctx() arms
(Task 1); synth signature extended with mut_scope_stack threaded
through ~19 recursive call sites in lib.rs plus the two re-synth
sites in mono.rs:712/1337 (Task 2); Term::Var resolution prepended
with innermost-first mut-scope lookup (Task 3); Term::Mut arm
(Task 4) and Term::Assign arm (Task 5) replacing the iter mut.1
Internal stubs with real logic; five negative fixtures + driver
(Task 6); positive verification of examples/mut.ail typechecking
clean (Task 7).
Four Boss decisions encoded: fixtures under examples/ with driver
at crates/ailang-check/tests/ (convention wins over spec wording);
mut_scope_stack as a synth parameter not an Env field (matches the
locals/effects/subst threading convention); MutAssignOutOfScope's
available list flattens all frames innermost-first;
mono.rs:712/1337 the two re-synth sites that need the threading.
Brummel
3a5b1d3347