fix: avoid ninja object path collisions

[aman-sharma-dev]

Summary

Fix Ninja object-file path collisions when source files in the same target have the same basename but different extensions.

Previously, sources such as src/start.c and src/start.S both mapped to src/start.o, causing Ninja to reject the generated build graph due to duplicate outputs.

Object paths now preserve the source extension, producing start.c.o and start.S.o.

Type of Change

• feat — New feature
• fix — Bug fix
• docs — Documentation only
• style — Formatting, no code change
• refactor — Code restructuring without behavior change
• test — Add or fix tests
• build — Build system or dependency changes
• ci — CI/CD pipeline changes
• perf — Performance improvement

Changes

• Preserve source extensions when generating Ninja object-file paths.
• Add regression coverage for same-basename sources with different extensions and generated output-path consistency.

Testing

• Unit tests pass (ctest --test-dir build --output-on-failure)
• Integration tests pass
• Manual testing performed
• New tests added for new functionality

Focused Ninja backend tests: 19 passed.

Ninja dry-run validation (ninja -n): passed.

Full Python test suite: 667 passed, 10 failed, 7 skipped. The 10 failures were reproduced against the original backend and are unrelated to this change.

Pre-Submission Checklist

• Code compiles without warnings (-Wall -Wextra -Werror for C)
• All existing tests pass
• New tests added for new functionality
• Documentation updated if API changed
• Commit messages follow convention
• Branch is rebased on latest master

Related Issues

N/A

Screenshots / Logs

N/A

Additional Notes

The full-suite failures are pre-existing and reproduce without this change.