Allow labeled arguments in any order for inferred function types

[cristianoc]

Removes the legacy commutable flag from Tarrow, together with its plumbing (commu_repr, set_commu, copy_commu, the Ccommu undo-log entry) and the Incoherent_label_order error.

Previously, arrows created while inferring a function's type from its call sites were marked Cunknown, and the argument-commuting loop in type_args refused to run on them. So a lambda parameter g first called as g(~a=1, ~b=2) could not later be called as g(~b=3, ~a=4):

let f = g => (g(~a=1, ~b=2), g(~b=3, ~a=4))
// error: This labeled function is applied to arguments
// in an order different from other calls. ...

Now labeled arguments commute at every call site regardless of whether the function type was declared or inferred.

Why this is sound

ReScript compiles labeled arguments positionally, in the order the parameters appear in the function's type. Reordering stays safe because:

1. Each function value has exactly one arrow order — the first call site fixes the inferred arrow order (and arity, via the existing force_tvar path).
2. Unification never commutes labels — unify3 still requires same_arg_label position by position, so any concrete function passed in must have been defined with its parameters in exactly the inferred order. Passing (~b, ~a) => … where (~a, ~b) => 'r was inferred remains a type error.
3. Every call site normalizes to the type's order — type_args walks the type's arrows and picks the matching syntactic argument by name, so the generated JS always passes arguments in parameter order (g(~b=3, ~a=4) compiles to g(4, 3)).

The commutable flag guarded principality (the inferred type of g depends on which call is typed first), not soundness — and this inference path already isn't principal in ReScript, since the arity itself is fixed by whichever total application is typed first. The lock was protecting a property the language had already given up, at the cost of a confusing error.

🤖 Generated with Claude Code

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.83%. Comparing base (11a25f4) to head (a7c4c90).

Files with missing lines	Patch %	Lines
compiler/ml/ctype.ml	61.53%	5 Missing ⚠️
analysis/src/type_utils.ml	63.63%	4 Missing ⚠️
compiler/ml/printtyp.ml	50.00%	2 Missing ⚠️
compiler/ml/record_type_spread.ml	0.00%	2 Missing ⚠️
compiler/ml/typecore.ml	88.88%	2 Missing ⚠️
compiler/ml/typedecl.ml	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8547      +/-   ##
==========================================
- Coverage   74.83%   74.83%   -0.01%     
==========================================
  Files         477      477              
  Lines       63313    63284      -29     
==========================================
- Hits        47382    47359      -23     
+ Misses      15931    15925       -6     

Files with missing lines	Coverage Δ
analysis/reanalyze/src/dead_optional_args.ml	92.00% <100.00%> (ø)
analysis/src/completion_back_end.ml	81.72% <100.00%> (ø)
analysis/src/completion_jsx.ml	75.00% <100.00%> (ø)
analysis/src/create_interface.ml	86.95% <100.00%> (ø)
analysis/src/shared.ml	67.44% <100.00%> (ø)
compiler/gentype/translate_type_expr_from_types.ml	89.09% <100.00%> (ø)
compiler/ml/btype.ml	82.44% <100.00%> (-0.08%)	⬇️
compiler/ml/translcore.ml	89.67% <100.00%> (ø)
compiler/ml/typeopt.ml	78.72% <100.00%> (ø)
compiler/ml/types.ml	66.66% <ø> (ø)
... and 8 more

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