docs(engine): add spec references for bytecode internals

[Xavrir]

Summary

• add module-level spec references for the bytecompiler and opcode layers
• document how spec ! operations map to Boa's internal bytecode invariants
• clarify operand decoding assumptions in the opcode argument helpers

Testing

• cargo fmt --all --check
• cargo check -p boa_engine --all-features
• pre-push cargo make run-ci

Closes #2426

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,073	50,073	0
Ignored	2,072	2,072	0
Failed	818	818	0
Panics	0	0	0
Conformance	94.54%	94.54%	0.00%

Tested main commit: 055ee0958ce332f3f99af27536a7c6f9a91def27
Tested PR commit: 15b421b555a4a9f92c4ce96417414d78ef49f67f
Compare commits: 055ee09...15b421b

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.51%. Comparing base (6ddc2b4) to head (15b421b).
⚠️ Report is 890 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5147       +/-   ##
===========================================
+ Coverage   47.24%   59.51%   +12.27%     
===========================================
  Files         476      580      +104     
  Lines       46892    63181    +16289     
===========================================
+ Hits        22154    37602    +15448     
- Misses      24738    25579      +841     

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