ci: run the -prod test suite on Windows as well

[metif12]

Problem

-prod removes assert statements, and that silently changes the behaviour of
any test that performs work inside one. In this repository it reached Windows as a
hang rather than a failure. Two tests did:

assert os.fd_dup2(transport.read_fd, 0) >= 0

The dup2 there is the operation the test depends on, not a check on it. Under
-prod the statement disappears, descriptor 0 keeps pointing at the real stdin,
and the read that follows blocks forever:

v -prod test lsp_test.v          -> never terminated (>40 min, vs 25s)
v -prod test integration_test.v -> likewise
v -prod test .                   -> never completed

Nothing caught it, because the -prod step only ran on Linux:

- name: Run tests with production optimizations
  if: runner.os == 'Linux'

Fix

1. Run the same step on Windows.

- name: Run tests with production optimizations (Windows)
  if: runner.os == 'Windows'
  timeout-minutes: 30
  env:
    V_MACOS_V3_NO_FALLBACK: "0"
  run: v -no-memory-limit -nocache -prod test vls/

"0" matches the existing Windows test step, since Windows defaults to V1 and 0
permits fallback.

2. Bound both -prod steps with timeout-minutes: 30. A hang in a test step
otherwise consumes the job's entire budget and reports nothing useful. A bounded
step turns it into a red build naming the step that stopped. The Linux step gets
the same guard because it has the same failure mode — drop that one line if you
would rather keep the diff Windows-only.

The Windows step deliberately does not set VLS_VLANG_V_REPO. That env var
enables the slow vlang/v workspace tests, which are already covered on Windows by
the non--prod step above, which does set it. Leaving it off keeps the added CI
time to roughly the 1-2 minutes the suite takes locally under -prod, instead of
the much longer run those tests add on top of -prod.

Validation

V 0137eb5 (the vlang/v revision CI builds from), Windows:

v -no-memory-limit -nocache -prod test .    ->  completes in 76s

76 seconds unbounded versus never terminating before #532. The 30-minute budget
is deliberately generous headroom over that.

I could not get a trustworthy figure for -prod combined with the vlang/v
workspace tests on this machine: another V build was running a full test .
concurrently and the two contended for all cores, so the number I got (over 50
minutes) reflects the contention, not the workload. That is exactly why the step
does not enable those tests.

Ordering constraint — please read before merging

This must land after #532. Before
that fix the two stdio tests above hang under -prod on Windows, so this step
goes red. With the timeout it fails in a bounded, legible way rather than
hanging, but it is still red.

The Windows job also needs #526 (the
suite must compile) and #528 (the job
currently dies at Check formatting) before it can be green at all.

Practical order: #526, #527, #528, #532, then this.

Related

#533 closes the matching codegen gap —
V3 was never compiled on Windows either — and has no ordering dependency.

[metif12]

Consolidated rather than merged on its own.

Folded into #536, which also carries #535's commits because master cannot build a green
suite without them. Once #535 lands, what remains here is this change.

Please review #536 instead. Nothing here is lost.

[metif12]

Closed in favour of #536: run the -prod suite on Windows.