Remove dead code enabled by structural arity

[cristianoc]

Part of the n-ary functions series tracked in #8557 (item 6; stacked on #8568 via base branch only — each PR merges independently).

What

Dead-code removal enabled by structural arity, plus one module deletion:

• The Too_many_arguments error cannot be raised anymore: the expected type is committed to an arrow of the literal's shape before destructuring, so every legacy path now surfaces as a regular type clash or Uncurried_arity_mismatch (which is what the fixture credited to it in ERROR_VARIANTS.md was already producing). Remove the variant, its printer, and the ?in_function threading through type_expect/type_cases that existed only to decorate it.
• Remove the function$-vs-arrow unification bridge in ctype (nothing produces a function$ type expression anymore) — and the bridge matched by ident name only, so a user-defined type \"function$" (escaped identifiers make this expressible) would have received its unsound cast rule; removing it closes that hazard, the structural arity counter Ctype.arity (no callers), the parsetree arity probes get_uncurry_arity/get_curry_arity (is_arity_one reads the params list directly), and Ast_async's redundant newtype double-dig.
• Deduplicate the arrow-flattening step shared by the analysis extract_function_type helpers.
• Delete Ast_compatible outright. Every member was a thin veneer over an existing Ast_helper constructor (rec_type_str/rec_type_sig had byte-identical signatures to Str.type_/Sig.type_), hand-rolling the records the canonical constructors build; the OCaml-version compatibility it existed for is long gone, and its fun_ silently hard-coded arity 1. Call sites use Ast_helper directly, with two let-bound app1/app2 shorthands kept local to ast_derive_js_mapper.

Generated code is unchanged. The Pjs_fn_make no-op elision explored alongside these cleanups is deliberately left out: it lands as the dedicated analysis in #8570 (item 7).

🤖 Generated with Claude Code

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[codecov]

Codecov Report

❌ Patch coverage is 95.74468% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.82%. Comparing base (63409eb) to head (2423f75).

Files with missing lines	Patch %	Lines
compiler/frontend/ast_core_type.ml	50.00%	1 Missing ⚠️
compiler/ml/typecore.ml	83.33%	1 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                     @@
##           codex/nary-typed-layers    #8569      +/-   ##
===========================================================
+ Coverage                    75.81%   75.82%   +0.01%     
===========================================================
  Files                          475      474       -1     
  Lines                        62900    62870      -30     
===========================================================
- Hits                         47686    47671      -15     
+ Misses                       15214    15199      -15     

Files with missing lines	Coverage Δ
analysis/src/type_utils.ml	63.99% <100.00%> (-0.05%)	⬇️
compiler/frontend/ast_comb.ml	100.00% <ø> (ø)
compiler/frontend/ast_core_type_class_type.ml	74.41% <100.00%> (ø)
compiler/frontend/ast_derive_abstract.ml	92.85% <100.00%> (ø)
compiler/frontend/ast_derive_js_mapper.ml	84.78% <100.00%> (+0.33%)	⬆️
compiler/frontend/ast_derive_projector.ml	90.62% <100.00%> (+0.14%)	⬆️
compiler/frontend/ast_exp_apply.ml	79.66% <100.00%> (ø)
compiler/frontend/ast_external_mk.ml	100.00% <100.00%> (ø)
compiler/frontend/ast_tdcls.ml	100.00% <100.00%> (ø)
compiler/ml/ast_async.ml	82.35% <100.00%> (-3.37%)	⬇️
... and 4 more

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commit: 2423f75