Eliminate Pjs_fn_make, Pjs_fn_make_unit, and unsafe_adjust_to_arity

[cristianoc]

Part of the n-ary functions series tracked in #8557 (item 7; stacked via base branch only — each PR merges independently).

What

With structural arity, "function of arity n" is a construction invariant rather than a goal state: translcore builds every Lfunction with exactly the parameters its type declares. The arity-enforcement layer therefore disappears:

• Function literals are emitted directly; the Pjs_fn_make wrapper that every function passed through was resolved as a no-op by lam_pass_alpha_conversion, but only after deep_flatten, simplify_exits and simplify_alias had run with the function hidden inside an Lprim, acting as an accidental optimization barrier.
• Pjs_fn_make_unit was a one-bit metadata channel: its entire effect was setting one_unit_arg so js_exp_make drops the unit parameter. translcore now sets the attribute directly, gated on the parameter pattern binding no identifiers (a () or _ pattern) — a more principled test than the alpha pass's check that the parameter was named "param".
• The active-pattern currying split in transl_function is deleted. It preserved pattern-effect timing across curried application steps, which no longer exist: total applications supply all arguments at once and explicit partial application eta-defers the entire call. The old output proves the point — the split's closures were immediately applied by the arity adapter, so only the allocations are gone (see mutable_uncurry_test).
• The I<N> unboxed-record producer (the @this method-callback encoding) is removed: the general Record_unboxed translation already returns the single field unboxed, and the wrapped value is a literal of matching arity.

With no producers left, both primitive constructors and every consumer arm are deleted, including the 230-line unsafe_adjust_to_arity (its only callers were the two Pjs_fn_make resolution sites).

Recursive modules

Removing the wrapper lets the static recursive-module compilation path see module members that are plain functions, replacing the Primitive_module.init/update bootstrap with hoisted function declarations. This is safe because the static path's own applicability check now sees the functions it was designed to check — the wrapper was hiding them, pessimizing compilation — and the bootstrap demonstrably remains for members that are not plain functions (rec_module_test keeps its lazy/value cases dynamic).

Review notes

• Removing the wrapper moves beta reduction of immediately applied functions five passes earlier (verified with -debug-ir). A follow-up in this PR keeps the beta-residue let chains local in Lam_pass_deep_flatten so Lam_pass_lets_dce can still substitute single-use arguments into the call — corpus impact: exactly one snapshot (a_recursive_type.mjs), restored to master's inline form.
• The wider inliner exposure also surfaced a pre-existing argument-evaluation-order bug in Lam_beta_reduce (reproducible on master, visible in bs_set_int_test.mjs's checked-in output); fixed separately in Fix argument evaluation order under function inlining #8572.

Verification

Stdlib byte-identical; full test suite green; JS output changes limited to removed adapter closures, removed no-op module bootstraps, better name preservation, and constant propagation.

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Codecov Report

❌ Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (codex/nary-dead-code@00a47ac). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
compiler/core/lam_primitive.ml	0.00%	2 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##             codex/nary-dead-code    #8570   +/-   ##
=======================================================
  Coverage                        ?   75.89%           
=======================================================
  Files                           ?      474           
  Lines                           ?    62772           
  Branches                        ?        0           
=======================================================
  Hits                            ?    47640           
  Misses                          ?    15132           
  Partials                        ?        0           

Files with missing lines	Coverage Δ
compiler/core/lam.ml	72.26% <ø> (ø)
compiler/core/lam_analysis.ml	66.91% <100.00%> (ø)
compiler/core/lam_compile.ml	85.42% <ø> (ø)
compiler/core/lam_compile_primitive.ml	75.49% <ø> (ø)
compiler/core/lam_convert.ml	79.32% <ø> (ø)
compiler/core/lam_eta_conversion.ml	0.00% <ø> (ø)
compiler/core/lam_pass_alpha_conversion.ml	77.35% <ø> (ø)
compiler/core/lam_pass_deep_flatten.ml	95.55% <100.00%> (ø)
compiler/core/lam_print.ml	12.13% <ø> (ø)
compiler/ml/lambda.ml	70.37% <ø> (ø)
... and 4 more

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commit: 0fce1b4