v0.3.0

Add macOS builds to the release workflow (#2)

* Add macOS builds to the release workflow

Cross-compile darwin/amd64 (Intel) and darwin/arm64 (Apple Silicon) alongside
the existing Windows and Linux targets — the project is pure Go with CGO off,
so these build cleanly. Include them in checksums.txt and the release assets,
and note macOS in the README download sections.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LqhckEzdTaofeg9tjiwo8G

* Clarify README download wording for pending macOS assets

The current latest release (v0.2.0) predates the macOS build steps, so "attached
to each GitHub Release" would briefly mismatch releases/latest until the next
tag. Reword to "New releases include ..." (EN) / "今後のリリースには ...
添付されます" (JP) so it stays accurate in the interim. Docs only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LqhckEzdTaofeg9tjiwo8G

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Co-authored-by: Claude <noreply@anthropic.com>