fix: migrate Phase-1 live check off tabbed Notepad onto mspaint (#20)

[CameronCrow]

Closes #20

Summary

• Replaces tests/test_notepad_live.py with tests/test_mspaint_live.py. Current Windows 11 notepad.exe is the single-process tabbed Notepad, so PID-scoped ownership (Phase 2A: PID-scoped window ownership #12) cannot launch-and-own it — app_session/ui.window(pid=...) fails safe there rather than risk attaching to one of Cameron's real open tabs.
• mspaint is already the live target elsewhere in the suite (test_app_session.py, test_acceptance_phase2.py), so this makes the Phase-1 mechanism proof ownable again on current Win11.
• Preserves the exact discipline the Notepad version demonstrated: act→verify round trip with independent re-reads, re-assert after actions (old state truly cleared, not appended to), right-window-among-several, and a real modal save-prompt asserted structurally and dismissed non-destructively.
• Every assertion is grounded in mspaint's real accessibility tree, captured live on Windows 11 while writing the check (not guessed) — see the new file's module docstring for what was tried and dropped (the Zoom text field, which lives inside a collapsed combo box and was flaky to resolve/act on) versus what shipped (Pencil/Eraser toggle-button checked,pressed states — the exact vocabulary ui.py's own docstring already cites as the original smoke-test finding — and a canvas click, which is what actually dirties a Paint document).
• Fixes a stray docstring reference in test_app_session.py that pointed at test_notepad_live and mis-described its own live test as launching Notepad (it already launches mspaint).
• Updates planning/TODO.md's Phase-1 line and planning/PLAN_MAIN.md's Phase-3 status note to reflect the migration — no other phase lines touched.

Test plan

• python -m pytest -q — 233 passed, 6 deselected (the live tests, as expected without -m live)
• python -m pytest tests/test_mspaint_live.py -m live -v — all 4 tests passed against a real Windows 11 desktop, run twice for stability, no stray mspaint.exe process or window left behind either time

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