Document Windows canvas and sidebar controls

[coneilen]

Summary

Adds visible Windows Help menu guidance for the existing canvas pan/zoom and sidebar root-loop reorder interactions. This improves the Keyboard discovery residual while the parity row remains Partial pending native foreground evidence.

Changes

• Append disabled informational labels for canvas pan/zoom and sidebar root-loop reordering after the existing shortcut guidance.
• Extend the hidden-HMENU contract to assert the exact interaction text, disabled state, and positive exact item count.

Test plan

RED: & $env:GRAPHCODE_ZIG0152 test src\MainWindow.zig -target x86_64-windows-msvc -lc -luser32 -lgdi32 -ladvapi32 -Iwinghostty\include --test-filter "main and help menus expose shortcuts and interaction guidance" -> 0 passed; 1 failed because the Canvas guidance label was empty
GREEN: & $env:GRAPHCODE_ZIG0152 test src\MainWindow.zig -target x86_64-windows-msvc -lc -luser32 -lgdi32 -ladvapi32 -Iwinghostty\include --test-filter "main and help menus expose shortcuts and interaction guidance" -> All 1 tests passed
REGRESSION: & $env:GRAPHCODE_ZIG0152 test src\MainWindow.zig -target x86_64-windows-msvc -lc -luser32 -lgdi32 -ladvapi32 -Iwinghostty\include -> All 53 tests passed

pwsh -NoProfile -File Tools\windows\validate.ps1 -Task windows-shell -SkipTrayLive -SwiftExecutable $env:GRAPHCODE_SWIFT633 reached 4/4 isolation-contract cases, 202/202 shell tests, and 29/29 worktree-status tests, then stopped before the Worktree Git process fixtures because this checkout projected a 307-character Git metadata path over the supported 259-character limit.

This is source and hidden-window contract evidence only; native foreground usability was not validated.

Checklist

• I have read the Contributing Guidelines
• I have signed off my commits (git commit -s) per the DCO
• Tests pass locally (make test)
• Code follows the existing style (make check)
• I added the test/contract before the implementation and observed the intended RED failure