js: uniform this-parameter for plain functions (callback thisArg)

[hiett]

Uniform this parameter for plain functions (callback thisArg)

A plain (non-method) function's this was baked to globalThis at lower time, so
it could never be a call-time value. That made every this-arg / *-5-* cluster in
test262's Array iteration methods fail (this === thisArg for a named callback), and
dropped the receiver on f.call/apply/bind.

Approach — bake this at the value boundary (no FFI ABI change)

Rather than widening the whole call ABI, this mirrors how class methods already
work: this is an explicit leading parameter of the implementation, and is baked
as a capture at the value boundary — so every callable BEAM value keeps its
user-arity and call_cb / apply_fn / func_* are unchanged.

• A top-level function whose body reads this is lowered as <name>%this/(n+1)
  taking this first, with the exported <name>/n a thin wrapper that forwards the
  sloppy-mode default this (the globalThis singleton). A plain f(…) call is
  therefore behaviour-identical.
• An Array iteration method (map/filter/forEach/some/every/find*) with a
  named callback + thisArg bakes the thisArg as the callback's this.
• f.call(t, …) → direct call of f%this; f.apply(t, arr) → this-baked closure
  applied over arr; f.bind(t, …) → closure with this fixed to t.
• Arrows are untouched (they correctly ignore a thisArg — lexical this).

Landed in two green stages: (1) the behaviour-preserving %this+wrapper split, then
(2) the thisArg wiring.

Results (test262, absolute-path suite)

Buckets: Array map/forEach/filter/every/some + Map/Set forEach.

	before	after
PASS (loaded+executed)	261	306 (+45)
fail_assert	65	20

• 0 regressions across those 7 buckets and across Function.prototype
  call/apply/bind + find/findIndex (checked pass→fail set-diff: none).
• The +45 are the Array -5- this-arg cluster (named callback asserting
  this === thisArg).

Tests

12 spec-driven tests appended under // ==== this parameter (wave 14) ====
(inline & named callback thisArg, f.call/.apply/.bind with a receiver, arrow
in a method sees the method's this, and a plain call's this still globalThis).

Green

Full gleam test: 2821 passed, 3 failures — the 3 are the pre-existing WASM
spec-suite failures (spec_suite_safe/unsafe, skip_count), identical to the base
branch. gleam format --check, gleam build (no warnings), and the erlc -Wall
FFI gate all clean.

Covered vs deferred

• Covered: named-callback thisArg for Array iteration; f.call/apply/bind(t, …)
  on a named this-reading function; inline-callback thisArg (wave 12, preserved);
  arrows ignore thisArg.
• Deferred (documented v1 gaps): a thisArg for a callback held in a VARIABLE
  (a function-expression value or let g = f; g.call(t) — the value carries no
  overridable this); Map/Set forEach thisArg (its test262 files use a variable
  callback → the same value gap); sloppy-mode this→global and primitive-this
  boxing for a directly-invoked function; bound-function .length/.name.

Merge-conflict surface

Touches function-lowering + call-site dispatch in
src/twocore/frontend/js/lower.gleam: lower_function (now returns
#(exported, extras)), program, lower_class (statics), lower_instance_method
and lower_instance_method_thisarg (new interception + lower_named_call/
_apply/_bind/_this_iter), plus a new Ctx.this_fns field. Also adds
apply_arg_list to rt_js.gleam, the FFI export list, and the emit js-op allowlist.