Fix unsoundness in JsValueStore by replacing unsafe raw pointer mutation of Arc with safe OnceLock

[mansiverma897993]

This Pull Request fixes/closes #5392.

It changes the following:

• Fix unsoundness in JsValueStore by replacing unsafe raw pointer mutation of Arc with safe OnceLock

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,125	53,125	0
Passed	51,073	51,073	0
Ignored	1,482	1,482	0
Failed	570	570	0
Panics	0	0	0
Conformance	96.14%	96.14%	0.00%

Tested main commit: 93a9e31a83bbaa15bbd8b687e61639ffc53bbef1
Tested PR commit: 357b23d64961869634715e7956f37de50ab45956
Compare commits: 93a9e31...357b23d

[codecov]

Codecov Report

❌ Patch coverage is 80.64516% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.81%. Comparing base (6ddc2b4) to head (357b23d).
⚠️ Report is 1019 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/module/loader/mod.rs	0.00%	3 Missing ⚠️
core/runtime/src/fetch/fetchers.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5408       +/-   ##
===========================================
+ Coverage   47.24%   62.81%   +15.57%     
===========================================
  Files         476      530       +54     
  Lines       46892    59108    +12216     
===========================================
+ Hits        22154    37130    +14976     
+ Misses      24738    21978     -2760     

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[hansl]

Why is the macro change part of this PR? Feels like it should be its own PR, can you split them?

[mansiverma897993]

Thanks for the review, @hansl.
I've split the PR as requested the macro change is no longer part of this PR. I removed the fix: allow deriving both TryFromJs and TryIntoJs with custom converters commit (that change was already merged separately in #5403), leaving only the JsValueStore OnceLock fix.
I also rebased onto the latest main, which resolves the conflicts from the store module move (core/runtime/src/store → core/wintertc/src/store).
All checks pass locally: boa_wintertc and boa_runtime tests, clippy with -D warnings, and cargo fmt --check.