fix(tinytorch): stop module start/resume dead-ending on tracking/disk desync

[Shashank-Tripathi-07]

Summary

tito module start and tito module resume can deadlock each other when .tito/progress.json's tracking (started_modules) desyncs from the actual notebook on disk under modules/. Each command's own failure message points at the other, and neither ever mentions the actual fix.

Area

• Book (textbook content, figures, exercises)
• StaffML (interview questions, challenges)
• Kits (hardware labs)
• Infrastructure (CI/CD, scripts, config)
• TinyTorch (modules, tests, milestones)

Changes

• tito/commands/module/workflow.py: start_module now checks whether the notebook directory actually exists before trusting the is_module_started flag, and recreates it from src/ when it doesn't, instead of refusing with 'already started'.
• resume_module now does the same check before calling _open_jupyter, recreating the notebook from src/ instead of dead-ending inside _open_jupyter with 'directory not found'.
• The normal case (notebook genuinely present) is unchanged in both commands.

Testing

• Rendered the book locally (quarto render)
• Ran tests (pytest tests/)
• Ran tito module test NN for affected module(s)
• Manual verification (describe below)

Reproduced the exact desync on a real, install.sh-created TinyTorch environment: started module 01, then deleted modules/01_tensor/ while leaving .tito/progress.json untouched (the same state tito system reset --keep-progress produces). Before the fix: tito module start 01 refused with 'already started', tito module resume 01 then failed with 'Module directory not found: 01_tensor', a closed loop with no escape mentioned in either message. After the fix: both commands detect the missing notebook, print a clear 'was started before, but its notebook is missing, recreating it from source' message, regenerate the notebook via the same _create_module_from_src path used for a genuinely new module, and proceed normally (resume goes on to open Jupyter Lab, confirmed via a real running server). Also confirmed the untouched case: starting an already-started module whose notebook genuinely still exists still correctly refuses and points to resume, unchanged from before.

Related Issues

None filed. Found while auditing every tito command against a real, isolated TinyTorch install to check which work correctly.

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[Shashank-Tripathi-07]

Superseded by #2092, which combines this with #2026 since both touch the same file (tito/commands/module/workflow.py). Closing to avoid two PRs churning the same file back to back.