fix(mlsysim): surface load errors via last_load_error on DesignLedger

[ShivtejG236]

Summary

Fixes #1994. DesignLedger.load()/load_async() previously reset silently
to a blank LedgerState() on any read failure (missing file, corrupt JSON,
IndexedDB error), with no way for calling code to distinguish "first run,
nothing saved yet" from "a save exists but couldn't be read" — a silent
data-loss bug on the read path. Adds last_load_error, mirroring the
last_save_error pattern #1988 established on the write path.

Area

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

(No box above quite fits mlsysim/Co-Labs specifically — checking
Infrastructure as the closest match. Actual area: mlsysim/mlsysim/labs/
and labs/tests/.)

Changes

• mlsysim/mlsysim/labs/state.py: add _last_load_error + read-only
  last_load_error property; set it in load() and load_async()'s
  except blocks instead of discarding the exception.
• mlsysim/tests/test_state.py: add native-path regression tests
  (missing file is not an error, corrupt file sets last_load_error and
  falls back to blank state, error clears on next successful load).
• labs/tests/test_wasm_persistence.py: add two real-browser
  (Playwright + Chromium + Pyodide) regression tests for load_async() —
  corrupt IndexedDB record, and indexedDB.open() throwing synchronously —
  reusing the served_dir fixture fix(labs): stop silently dropping WASM DesignLedger saves (#1985) #1988 introduced.

Rebased cleanly onto dev after #1988 merged; no leftover conflicts.

Testing

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

Ran explicitly:

python -m pytest mlsysim/tests/test_state.py -v      # 11 passed
python -m pytest labs/tests/test_wasm_persistence.py -v   # 3 passed (real Chromium)

[Shashank-Tripathi-07]

lgtm. tagging professor for a review from his end.

[profvjreddi]

Thanks @ShivtejG236, and welcome. Integrated into dev.

Your six new tests pass here alongside the existing five. Good first PR to pick, and thanks for the patience on the review.

I applied your commit onto a review branch off current dev rather than merging the PR head, so GitHub will show this as closed instead of merged. Authorship and credit are intact.

@all-contributors please add @ShivtejG236 for bug, code, test in mlsysim

[github-actions]

I've added @ShivtejG236 as a contributor to mlsysim! 🎉

Recognized for: bug, code, test
Project(s): mlsysim (explicitly mentioned in comment)
Based on: @all-contributors please add @ShivtejG236 for bug, code, test in mlsysim

The contributor list has been updated in:

• mlsysim/.all-contributorsrc, mlsysim/README.md
• Main README.md

We love recognizing our contributors! ❤️

[ShivtejG236]

Thanks @ShivtejG236, and welcome. Integrated into dev.

Your six new tests pass here alongside the existing five. Good first PR to pick, and thanks for the patience on the review.

I applied your commit onto a review branch off current dev rather than merging the PR head, so GitHub will show this as closed instead of merged. Authorship and credit are intact.

@all-contributors please add @ShivtejG236 for bug, code, test in mlsysim

Thank you, Professor!

Was an amazing experience working on this. As my first real OSS contribution, I learnt a lot, especially through the review process

[github-actions]

I've added @ShivtejG236 as a contributor to mlsysim! 🎉

Recognized for: bug, code, test
Project(s): mlsysim (explicitly mentioned in comment)
Based on: > @all-contributors please add @ShivtejG236 for bug, code, test in mlsysim

The contributor list has been updated in:

• mlsysim/.all-contributorsrc, mlsysim/README.md
• Main README.md

We love recognizing our contributors! ❤️

[github-actions]

I've added @ShivtejG236 as a contributor to mlsysim! 🎉

Recognized for: bug, code, test
Project(s): mlsysim (explicitly mentioned in comment)
Based on: > @all-contributors please add @ShivtejG236 for bug, code, test in mlsysim

The contributor list has been updated in:

• mlsysim/.all-contributorsrc, mlsysim/README.md
• Main README.md

We love recognizing our contributors! ❤️