runtime: fix String.fromCharCode wrapping and fromCodePoint RangeErrors

[andrewtdiz]

Summary

Perry has runtime helpers for String.fromCharCode and String.fromCodePoint, and HIR already expands multi-argument valid calls. The remaining edge semantics diverge from Node:

• String.fromCharCode should convert each argument with ToUint16, so negative / out-of-range values wrap modulo 65536 instead of returning "".
• String.fromCodePoint should throw RangeError for invalid code points, negative values, and non-integers instead of returning "".
• Zero-argument calls should return "".

Node behavior

Local Node v25.9.0 probe:

fromCharCode:[65,66,67]="ABC":41+42+43
fromCharCode:[128512]="":f600
fromCharCode:[-1]="�":ffff
fromCharCode:[65.9]="A":41
fromCharCode:[]="":
fromCodePoint:[128512,65]="😀A":1f600+41
fromCodePoint:[1114112]=RangeError:Invalid code point 1114112
fromCodePoint:[-1]=RangeError:Invalid code point -1
fromCodePoint:[3.14]=RangeError:Invalid code point 3.14
fromCodePoint:[]="":

Probe shape:

String.fromCharCode(65, 66, 67)
String.fromCharCode(0x1F600)
String.fromCharCode(-1)
String.fromCharCode(65.9)
String.fromCharCode()
String.fromCodePoint(0x1F600, 65)
String.fromCodePoint(0x110000)
String.fromCodePoint(-1)
String.fromCodePoint(3.14)
String.fromCodePoint()

Perry evidence

• crates/perry-hir/src/lower/expr_call/module_static.rs handles one-or-more String.fromCharCode(...) / String.fromCodePoint(...) arguments by lowering each argument to a single-code helper and concatenating multi-argument calls.
• crates/perry-codegen/src/expr/string_regex_proc.rs lowers both helpers by converting the argument with raw fptosi(DOUBLE -> I32) before calling the runtime helper.
• crates/perry-runtime/src/string/char_ops.rs::js_string_from_char_code() returns an empty string for values outside 0..=0xFFFF instead of applying ToUint16 wrapping.
• crates/perry-runtime/src/string/char_ops.rs::js_string_from_code_point() returns an empty string for values outside 0..=0x10FFFF / invalid scalar values instead of throwing RangeError.
• test-files/test_gap_string_methods.ts covers valid multi-argument fromCodePoint and zero, but not invalid code points or fromCharCode wrapping.

Duplicate check

Searched issues and PRs for:

• String.fromCharCode variadic
• String.fromCodePoint RangeError
• fromCharCode fromCodePoint
• PR search for fromCharCode OR fromCodePoint OR String.fromCodePoint

No focused tracker showed up. PR #1043 was a WASM-specific fix for String.fromCharCode returning undefined, not these runtime edge semantics.

Expected fix shape

• Implement ToUint16 semantics for String.fromCharCode, including negative and out-of-range wrapping.
• Make String.fromCodePoint validate integer code points and throw RangeError for negative, non-integer, and > 0x10FFFF values.
• Avoid raw fptosi for non-finite / non-integer values where JS coercion or validation must be observable.
• Ensure zero-argument static calls return "".
• Add regression coverage for wrapping, invalid code points, non-integers, and zero-argument calls.