fix(runtime): small built-ins tails (Date setters/parse, BigInt ++/--)

[proggeramlug]

Summary

Mops several small high-% built-in tails (Date + BigInt) from the test262 sweep over built-ins/{Number,Boolean,Symbol,Error,Map,Set,Weak*,Reflect,BigInt,Math,Date,ArrayBuffer,URI,parse*,global}.

Parity 93.4% → 95.0% over 2314 judged tests: +38 fixed, 0 regressed (Date 37, BigInt 1), verified by diffing failure sets against a from-scratch build of the merge base (aee4e82c5).

Root causes & fixes

1. Date set* coercion order (date.rs::js_date_apply_setter) — thisTimeValue must be read before coercing any argument, then all present arguments coerced left-to-right before the NaN-time short-circuit. The code did the opposite (read-AFTER the leading arg, short-circuited before coercing the rest). node v26 confirms read-before / coerce-all. Fixes set{Date,Hours,Minutes,Seconds,Milliseconds,Month,FullYear,UTC*}/{date-value-read-before-tonumber-*,arg-coercion-order} (~22).

2. Date expanded-year ISO parse (date.rs::parse_iso8601) — accept the ±YYYYYY extended-year form (mandatory sign, 6 digits; -000000 rejected) in addition to 4-digit YYYY. new Date('-000001-07-01T00:00Z') no longer yields Invalid Date, fixing the {toString,toDateString,toUTCString}/negative-year set.

3. Date toISOString expanded-year (iso_year) — years outside 0..=9999 serialize as ±YYYYYY (new Date(8.64e15).toISOString() → +275760-09-13T00:00:00.000Z).

4. Date.UTC / constructor year rebasing (rebase_two_digit_year) — apply ToInteger (truncate-toward-zero) before the 0..=99 two-digit-year test, so Date.UTC(-0.999999, 0) rebases to 1900, not literal year 0.

5. Date.parse TimeClip — a parsed instant outside ±8.64e15 ms is now Invalid (Date.parse("-271821-04-19T23:59:59.999Z") → NaN).

6. BigInt-preserving ++/-- (dynamic_arith.rs + codegen literals_vars.rs) — the update slow path coerced the operand with js_number_coerce + fadd 1.0, collapsing a BigInt to a Number (let i = 10n; i++ produced the Number 11, so a later i + 87n threw a mixed-type TypeError). New js_to_numeric (ToNumeric: BigInt passthrough, else ToNumber) + js_numeric_step (steps by 1n/1.0 of the operand's own type). Non-BigInt behavior is byte-identical; integer-local fast path (fadd) unchanged. Fixes BigInt/prototype/toString/a-z.

Files

• crates/perry-runtime/src/date.rs
• crates/perry-runtime/src/value/dynamic_arith.rs
• crates/perry-codegen/src/expr/literals_vars.rs
• crates/perry-codegen/src/runtime_decls/strings.rs

Validation

• Built + tested on a 48-core box against test262 (node v26 oracle), --jobs 40.
• Zero-regression proof: failure-set diff vs a clean build of merge base aee4e82c5 → 38 fixed, 0 regressed.
• No Cargo.toml/CLAUDE.md/CHANGELOG.md edits (maintainer folds version/changelog at merge).