Require windows-spikes validation for token and contract changes

[coneilen]

Summary

Close a gap in the orchestration plan's per-PR gate. A wave-1 PR added a design token, passed the full windows-shell unit run, and then failed exact-head CI. The failure was in the palette-parity contract, which runs only under validate.ps1 -Task windows-spikes.

Changes

• investigation/windows-parity-orchestration-plan.md: the per-PR full-validation step now also requires -Task windows-spikes when DesignTokens.zig, palette, font or contract files change.

Test plan

RED: exact-head CI on PR #789 (e6fcc49), Windows port validation run 38525363547 -> PaletteParity.Tests.ps1 failed: parsed Windows token has no mapping or documented exception: loop_minimap_surface, after the lane's full -ShellPart unit run passed 55/55 sections
GREEN: Select-String investigation\windows-parity-orchestration-plan.md -Pattern 'windows-spikes' -> the per-PR gate names it for token/palette/contract changes
REGRESSION: git diff --check -> clean; git ls-files --eol -> CRLF preserved; bash Tools/ci/tests/classify-changes.test.sh -> 59 passed, 0 failed

Documentation-only change: no product code, so the Windows suites and macOS make test/make check were not run.

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