Make web lab tests use explicit UTF-8 encoding

[qiann0512-gif]

Summary

• read web lab HTML and SCENES.md fixtures with explicit UTF-8 encoding
• write the temporary node --check module as UTF-8
• fixes Windows locale-dependent UnicodeDecodeError/UnicodeEncodeError in test_web_lab.py

Tests

• pytest tests/test_web_lab.py -q (34 passed)
• pytest tests -q (173 passed, 56 skipped, 1 warning)

[abhicris]

🤖 Audit verdict: needs_human

PR title claims UTF-8 encoding changes but actually removes substantial test coverage (simulator test, multipage lab checks, home link test refactoring) without justification in scope.

Risks flagged:

• Test coverage regression: removal of test_simulator_links_to_multipage_lab() and test_files_exist() MULTIPAGE_LAB/LAB_PAGES checks not explained
• Scope creep: PR titled 'explicit UTF-8 encoding' includes infrastructure refactoring (runnable_node removal, test consolidation) that should be separate
• test_home_links_to_lab() changed from checking dual links to single link—unclear if intentional

Audited by the kcolbchain PR pipeline. See pipeline docs.