Fix %jigsaw cross-origin errors and restore button functionality

[blink1073]

Closes #176, #199, #170, #166, #165

Summary

• Replace all window.parent.* direct-access calls in the downloaded iframe HTML with postMessage, fixing cross-origin/sandboxed frame errors in modern browsers
• Restore Run, Generate Python Code, and Clear Cell Output button behaviour for both Notebook 6 and Notebook 7
• Embed previously saved workspace XML in the HTML at magic run time so Blockly state is restored when the cell is re-executed (no XHR needed)
• Show run output and generated code in the jigsaw cell's own output area rather than a separate cell
• Fix Blockly code generator indentation from 2 spaces to 4 spaces
• Create subdirectory for workspace path when it includes a path component (already fixed in Fix %jigsaw failing to save files when workspace path includes a subdirectory #374, now included here for completeness)

Technical approach

The old implementation relied on window.parent.Blockly, window.parent.IPython, and window.parent.document.* — all blocked by modern browser security for cross-origin or null-origin sandboxed frames.

The new implementation:

1. _fix_cross_origin() rewrites the downloaded Blockly HTML to remove all window.parent.* accesses, inject saved workspace XML as a JS variable, and replace button onclick handlers with local postMessage helpers
2. Parent-side listener (injected via Javascript()) receives execute/insert/clear messages from the iframe and handles them using the appropriate notebook API — the classic IPython.notebook OutputArea API for Notebook 6, or DOM manipulation + Jupyter REST/WebSocket for Notebook 7
3. Cell caching stores the owning cell object at magic run time (while element is in scope) so the message handler can find it later without frameElement, which is null for cross-origin frames

Test plan

• Run just test tests/magics/test_jigsaw_magic.py — both tests pass
• Manually tested in Notebook 6: Run executes code and shows output in jigsaw cell, Generate shows code block, Clear removes both
• Manually tested in Notebook 7: same behaviour via WebSocket execution and DOM output
• Manually tested in JupyterLab: same behaviour
• Verify workspace XML is restored after re-running the %jigsaw cell

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.82%. Comparing base (f42dd6e) to head (96ce86e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
metakernel/magics/jigsaw_magic.py	75.00%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #375      +/-   ##
==========================================
- Coverage   90.96%   90.82%   -0.14%     
==========================================
  Files          51       51              
  Lines        2921     2943      +22     
  Branches      409      410       +1     
==========================================
+ Hits         2657     2673      +16     
- Misses        185      190       +5     
- Partials       79       80       +1     

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