regexp: mitigate ReDoS on ASCII inputs with PikeVM path

[Flamki]

This Pull Request mitigates #4643.

It changes the following:

• Adds a dedicated non-optimized regress::Regex matcher for PikeVM execution in RegExp objects.
• Uses PikeVM matching for ASCII inputs in RegExpBuiltinExec, avoiding pathological backtracking in common ReDoS cases such as (a+)+$.
• Keeps existing UTF-16/UCS-2 backtracking paths for non-ASCII inputs to preserve current encoding semantics.
• Adds a regression test covering the nested-quantifier ASCII case.

Notes:

• This is a mitigation path inside Boa while the broader backtracking-budget API is still blocked in regress.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,862	52,862	0
Passed	49,504	49,503	-1
Ignored	2,262	2,262	0
Failed	1,096	1,097	+1
Panics	0	0	0
Conformance	93.65%	93.65%	-0.00%

Broken tests (1):

test/staging/sm/RegExp/unicode-raw.js (previously Passed)

[Flamki]

Follow-up fix pushed in 63d6611.

Addresses the CI failures from the previous run:

• Added explicit lifetime args for
  egress::backends::PikeVMExecutor<', '> at both ind call sites (fixes elided-lifetimes-in-paths under -D warnings).
• Updated doc comments to use backticks around PikeVM (fixes clippy::doc-markdown).
• Applied rustfmt-compliant line wrapping in the ASCII UTF-16 conversion path.

This should resolve the build/lint/fmt failures that were cascading across jobs.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.07%. Comparing base (6ddc2b4) to head (63d6611).
⚠️ Report is 688 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/regexp/mod.rs	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4698      +/-   ##
==========================================
+ Coverage   47.24%   57.07%   +9.82%     
==========================================
  Files         476      549      +73     
  Lines       46892    60311   +13419     
==========================================
+ Hits        22154    34421   +12267     
- Misses      24738    25890    +1152     

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[Flamki]

@jedel1043 I did a full cleanup pass on #4698 so it avoids the same issues we hit in earlier PRs.

rebased the branch onto latest main, kept the PikeVM ReDoS mitigation logic intact, and preserved the CI/lint fixes (explicit PikeVMExecutor<'_, '_> lifetimes, doc markdown fixes, and rustfmt-compliant formatting).

• also added one extra regression test to cover the ASCII-in-UTF16 execution path, not just the plain ASCII string path, so coverage is better aligned with the new matcher behavior.

Current branch head: 91d9da41.
Ready for CI + maintainer review.