Fix register forwarding when the right operand mutates the left local

[dev-willbird1936]

This Pull Request fixes/closes #5456.

The bytecode compiler can forward the register of a mutable local into a binary operation. It compiles the right operand before it copies the left value into the destination register. When the right operand mutates that local, the generated operation reads the mutated value instead of the value that left-to-right JavaScript evaluation captured, so let a = 1; a = a + (a = 5); produces 10 instead of the specified 6.

This change snapshots mutable local identifiers into a temporary register before the other operand is compiled, and keeps the existing fast path for immutable locals and cached constants. The regression test covers four mutating right-hand operands across arithmetic, bitwise, strict-equality, and relational operators. The existing boa_engine suite is the negative control for unchanged behaviour on ordinary binary expressions.

Validation on current main:

• cargo test -p boa_engine binary_operands_preserve_left_to_right_evaluation
• cargo fmt --all -- --check

[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: 4fc75c6ae9d85f2b8065c6716f88e9b35318438c
Tested PR commit: 5a469a6b4583d8a0479ee95514b2fd364157b292
Compare commits: 4fc75c6...5a469a6

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.93%. Comparing base (6ddc2b4) to head (5a469a6).
⚠️ Report is 1012 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5457       +/-   ##
===========================================
+ Coverage   47.24%   62.93%   +15.68%     
===========================================
  Files         476      530       +54     
  Lines       46892    59119    +12227     
===========================================
+ Hits        22154    37205    +15051     
+ Misses      24738    21914     -2824     

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