fix(engine): keep pending return value in a dedicated slot across finally

[xcb3d]

Related test262 case: test/staging/sm/Function/return-finally.js (nested try-finally where break discards an inner return).

It changes the following:

• Keep an explicit return value that passes through a finally block in a dedicated function-level register instead of the value stack, so abrupt completions inside finally cannot leave a stale value behind for an outer pending return.
• Save/restore the pending-return slot across finally blocks.
• Add a regression test for a nested return discarded by break, plus the return-in-finally override case.

Verification:

• test262 test/staging/sm/Function/return-finally.js passes (also fixes the same-named generators case with the same root cause).
• cargo test -p boa_engine --lib passes; cargo clippy and cargo fmt check pass.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,578	53,578	0
Passed	51,432	51,434	+2
Ignored	1,648	1,648	0
Failed	498	496	-2
Panics	0	0	0
Conformance	95.99%	96.00%	+0.00%

Fixed tests (2):

test/staging/sm/Function/return-finally.js (previously Failed)
test/staging/sm/generators/return-finally.js (previously Failed)

Tested main commit: 257bc301afa0cda88f470a8452251ade54d516e3
Tested PR commit: ae8beeda387c38ce0998e711e82c9a5a801f353e
Compare commits: 257bc30...ae8beed

[codecov]

Codecov Report

❌ Patch coverage is 91.83673% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.93%. Comparing base (6ddc2b4) to head (ae8beed).
⚠️ Report is 1052 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/bytecompiler/statement/mod.rs	81.81%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5514       +/-   ##
===========================================
+ Coverage   47.24%   62.93%   +15.68%     
===========================================
  Files         476      536       +60     
  Lines       46892    60320    +13428     
===========================================
+ Hits        22154    37960    +15806     
+ Misses      24738    22360     -2378     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.