fix(array): honor fromIndex/start/end/depth in indexOf/fill/flat/copyWithin

[proggeramlug]

Closes #2804
Closes #2801
Closes #2800
Closes #2802

Implementation

All four issues were optional-argument gaps where the runtime/dynamic-dispatch
path ignored a trailing argument the static codegen path already understood.

#2804 — indexOf / includes fromIndex (Array + TypedArray).
js_array_indexOf_jsvalue and js_array_includes_jsvalue now take
(value, from_index, has_from) and apply ECMA-262 ToIntegerOrInfinity +
clamping (new shared forward_start_index helper): omitted → 0, positive →
that index, negative → max(len + n, 0), NaN → 0, +Infinity → not-found.
Works for both Array and TypedArray receivers. The optional from_index was
threaded through the existing Expr::ArrayIndexOf / Expr::ArrayIncludes HIR
variants by adding a field (mirroring the existing
Expr::ArrayLastIndexOf) — no new HIR variant was added, so stable-hash
tags 254/255 are unchanged. Static codegen (lower_array_method, misc_methods,
instance_misc1), all HIR lowering construction sites, and the dynamic
native_call_method dispatch arms now pass the fromIndex.

#2801 — fill(value, start, end) dynamic dispatch. The "fill" arm in
native_call_method now routes 2+ args to js_array_fill_range (default
end = +Infinity → clamps to length, matching the static path) instead of
always whole-array js_array_fill.

#2800 — flat(depth) dynamic dispatch. The "flat" arm now routes a
supplied depth to js_array_flat_depth (NaN/≤0 → 0, +Infinity → fully flat),
falling back to legacy js_array_flat only when depth is omitted.

#2802 — copyWithin dynamic dispatch. Added a "copyWithin" arm mirroring
the static path: require target, default start to 0, pass has_end = 0
when end is omitted, mutate and return the receiver via
js_array_copy_within.

Drive-by: fixed a pre-existing duplicate stable-hash tag 12045 shared by
RegExpEscape and ReflectIsExtensible (introduced by parallel-merged PRs on
main) that was failing cargo test -p perry-hir
(expr_variant_stable_hash_tags_are_unique). Reassigned
ReflectIsExtensible to the next free tag 12048.

Validation

Gap test test-files/test_gap_array_optargs_2804_2801_2800_2802.ts exercises
each method via both direct calls and dynamic dispatch (computed method
name / as any receiver), covering positive/negative/NaN/Infinity fromIndex,
negative fill ranges, flat depth incl. Infinity, and copyWithin overlap.

Under the DEFAULT auto-optimize compile the output is byte-identical to
node --experimental-strip-types:

$ diff <(node --experimental-strip-types test_gap_array_optargs_2804_2801_2800_2802.ts) <(./out)
# (no output — identical)

• ./scripts/check_file_size.sh → OK (no file over 2000 lines)
• cargo fmt --all -- --check → clean
• cargo test --release -p perry-runtime array → 101 passed
• cargo test --release -p perry-hir → all pass (incl. stable-hash uniqueness)
• Full cargo build --release (cold) → green