refactor(engine): encapsulate Context VM field access behind wrapper methods

[hansl]

Add context/vm_access.rs with 45 wrapper methods that delegate to Context's internal Vm field, then replace all ~600 direct context.vm.* accesses across 78 files with calls to these wrappers.

This is the first step toward extracting Context into its own crate (boa_context). By routing all external access through methods, the internal structure of Context can be changed without touching call sites.

[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: 498b424578252944cbbfae90753e73f8c15149ae
Compare commits: 055ee09...498b424

[codecov]

Codecov Report

❌ Patch coverage is 76.82737% with 149 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.61%. Comparing base (6ddc2b4) to head (498b424).
⚠️ Report is 890 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/object/builtins/jspromise.rs	0.00%	16 Missing ⚠️
core/engine/src/builtins/proxy/mod.rs	0.00%	11 Missing ⚠️
core/engine/src/vm/opcode/call/mod.rs	47.36%	10 Missing ⚠️
core/engine/src/vm/opcode/iteration/iterator.rs	61.53%	10 Missing ⚠️
core/engine/src/vm/opcode/environment/mod.rs	67.85%	9 Missing ⚠️
core/engine/src/vm/opcode/define/class/setter.rs	20.00%	8 Missing ⚠️
core/engine/src/vm/opcode/set/private.rs	20.00%	8 Missing ⚠️
core/engine/src/vm/opcode/set/property.rs	66.66%	8 Missing ⚠️
core/engine/src/vm/opcode/control_flow/jump.rs	76.66%	7 Missing ⚠️
core/engine/src/vm/opcode/push/class/private.rs	0.00%	7 Missing ⚠️
... and 23 more

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5149       +/-   ##
===========================================
+ Coverage   47.24%   59.61%   +12.36%     
===========================================
  Files         476      581      +105     
  Lines       46892    63218    +16326     
===========================================
+ Hits        22154    37685    +15531     
- Misses      24738    25533      +795     

☔ View full report in Codecov by Sentry.
📢 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.