[v0.2.0] Add Windows CI lane (continue-on-error)

[neverDefined]

Problem

CI matrix today is (ubuntu, macos). Windows is intentionally skipped because Foundry's Windows story is weak. But the Go side of go-anvil compiles fine on Windows; only the runtime tests need anvil. Surfacing Windows compile breakage early is cheap.

Proposal

Add a separate Windows job that runs:

windows:
  runs-on: windows-latest
  continue-on-error: true   # informational only for now
  steps:
    - uses: actions/checkout@v6
    - uses: actions/setup-go@v6
      with:
        go-version: '1.26'
    - run: go build ./...
    - run: go test -short ./...   # only unit tests (httptest-backed)

The -short gate (added in #69) means anvil-requiring tests skip cleanly.

Acceptance criteria

• Job runs on every PR.
• Doesn't block merge (continue-on-error: true).
• Build + unit tests pass on Windows. If they don't, file a separate issue rather than fixing in-line.