Fix stale objects after source metadata collisions

[Daniiiil1]

Summary

• disable SCons' max_drift content-signature shortcut for PlatformIO builds
• add a regression test that replaces an old source file with different, same-size content while preserving its mtime
• verify the corrected build produces the new program and a subsequent unchanged build does not rewrite the object

Why

SCons normally avoids rehashing files older than its max-drift window when their stored size and timestamp still match. A Git checkout can replace a source while preserving both values, causing PlatformIO to combine objects from different source states. Setting max_drift to -1 keeps content-based dependency decisions but removes that unsafe metadata shortcut.

Fixes #5481.

Validation

• pytest -q tests/commands/test_run.py — 8 passed
• isolated test_ci_boards and test_calculator_example — 2 passed after initializing the repository's examples submodule
• make before-commit — codespell, isort, black, and pylint passed; pylint 10.00/10
• broader suite before submodule initialization — 103 passed, 15 skipped; the two missing-fixture failures were then reproduced and resolved by initializing examples

[CLAassistant]

All committers have signed the CLA.