fix(runtime): complete RegExp test262 worklist

[proggeramlug]

Summary

• Fixes test262 built-ins/RegExp — 89 (self-contained worklist) #5897 in full: every one of the 89 listed test262 RegExp cases passes.
• Add a narrowly selected ECMAScript backtracking fallback for quantified-group capture reset/retry semantics and lookaround backreferences, while keeping the existing linear and fancy engines as the common paths.
• Match and replace lone-surrogate JavaScript strings as UTF-16 code units and preserve WTF-8 storage.
• Add focused runtime regressions for every repaired semantic root cause. No version bump.

Validation

• Real scripts/test262_subset.py runner over the exact 89 issue paths at test262 4249661388e5d3f92a85186213da140a6481490f: 89 pass, 0 diff, 0 runtime-fail, 0 compile-fail, 0 skip. The exact list was run in eight disjoint shards to bound temporary compiler storage; reconciliation found 89 issue paths, 89 runner paths, 0 missing, and 0 extra.
• cargo test -p perry-runtime --lib: 2,649 passed, 0 failed, 4 ignored.
• CI-derived static lint tier: all 53 commands passed, including cargo fmt --all -- --check, the 2,000-line file-size cap, runtime GC-holder custody, raw-handle debt, unrooted-local shape, parity, and merge-base ratchets.
• git diff --check.

No tests from the issue worklist are excluded or structurally impossible.

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📥 Commits

Reviewing files that changed from the base of the PR and between 2382a9f and 55a5d03.

⛔ Files ignored due to path filters (1)

• Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (9)

• changelog.d/8667-regexp-repeat-matcher.md
• crates/perry-runtime/Cargo.toml
• crates/perry-runtime/src/regex.rs
• crates/perry-runtime/src/regex/compile.rs
• crates/perry-runtime/src/regex/exec.rs
• crates/perry-runtime/src/regex/exec_array.rs
• crates/perry-runtime/src/regex/match_string.rs
• crates/perry-runtime/src/regex/repeat_matcher.rs
• crates/perry-runtime/src/regex/tests.rs

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[proggeramlug]

Needs a rebase onto current main (7ad718ab4) — conflicts now, so I can't land it as-is.

The collision came from f5739b532 (the #8661 / #8656 / #8666 / #8662 / #8660 batch), which reworked the regex module, which I merged while this PR was open. Conflicting files:

• crates/perry-runtime/Cargo.toml
• crates/perry-runtime/src/regex.rs
• crates/perry-runtime/src/regex/exec.rs
• crates/perry-runtime/src/regex/match_string.rs
• crates/perry-runtime/src/regex/repeat_matcher.rs

The Cargo.toml conflict is worth a careful look rather than a mechanical union — if both sides moved a dependency, taking either side alone drops the other's.

Everything else in the PR auto-merges, so this should be a contained rebase. Ping me when it's pushed and I'll re-run the batch validation (9 ratchet gates + cargo fmt --check, perry-codegen --lib, perry-runtime --lib at RUST_TEST_THREADS=1) and land it.