cli: support BOA_TRACE env flag for VM trace mode

[Flamki]

Summary

This is a minimal, first-step observability improvement for #4937.

It adds an opt-in environment toggle for existing VM trace output so tracing can be enabled in non-interactive workflows (scripts/CI/wrappers) without changing source code.

Changes

• CLI: add BOA_TRACE support in addition to --trace.
• Accept strict boolean-like values for BOA_TRACE:
  • truthy: 1, true, yes, on
  • falsy: 0, false, no, off
• Error clearly on invalid/non-UTF8 BOA_TRACE values.
• Keep precedence deterministic: --trace overrides BOA_TRACE.
• Add focused unit tests for env var parsing.
• Update docs/debugging.md with usage examples and value rules.

Scope

• Intentionally narrow: CLI + docs only.
• No VM core tracing redesign in this PR.
• No metrics subsystem introduced yet.

Validation

• cargo fmt --all -- --check
• cargo test -p boa_cli tests::parse_bool_env_var -- --nocapture
• cargo clippy -p boa_cli --all-targets --no-deps -- -D warnings

Partially addresses #4937.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,713	49,713	0
Ignored	2,262	2,262	0
Failed	988	988	0
Panics	0	0	0
Conformance	93.86%	93.86%	0.00%

Tested main commit: dc02d4f354ac278dd845f529a3331001c4ea0ede
Tested PR commit: 34ec2da1c6640330b9e2e573ca152e8e4778e738
Compare commits: dc02d4f...34ec2da

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.35%. Comparing base (6ddc2b4) to head (34ec2da).
⚠️ Report is 830 commits behind head on main.

Files with missing lines	Patch %	Lines
cli/src/main.rs	55.55%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4982       +/-   ##
===========================================
+ Coverage   47.24%   58.35%   +11.11%     
===========================================
  Files         476      556       +80     
  Lines       46892    61171    +14279     
===========================================
+ Hits        22154    35697    +13543     
- Misses      24738    25474      +736     

☔ 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.

[jedel1043]

Kind of superseeded by #4931, and arguably much better since this assumes you have an OS to get env variables, but the other PR is customizable per the host's requirements.