Allow journald package to build on Windows

[vzer200]

Fixes #478. Follows the build-constraint removal proposed in #479, which was closed without merging.

The pinned go-systemd dependency already provides Windows stubs, but the !windows constraint excluded NewJournalDWriter when Windows applications imported this package. Remove that constraint and add a Windows regression for the unavailable-journal error and zero bytes written.

This enables compilation on Windows. Writing still returns the dependency's error because journald is unavailable there. No dependencies or existing writer logic change.

Validation on Windows/amd64:

• The new regression fails to compile on the original source and passes with this change.
• Go 1.23.12 and 1.24.13: both existing CI commands pass, including go test -race -bench . -benchmem ./... with default benchmark durations and go test -tags binary_log -race ./....
• Go 1.26.8: full normal and binary race suites and go vet ./... pass. A separate consumer verifies public-package imports, input preservation and logger error propagation in both modes.
• Formatting and diff checks pass. Existing Linux journal tests cross-compile; Linux tests were not run locally. Local race tests use LLVM-MinGW Clang, so the host is not identical to GitHub's Windows runner.

AI-assisted implementation with independent agent review of the final source and test evidence before submission.