ci(build): add linux/386, linux/arm, windows/arm64 to build.yml compile matrix

[saadqbal]

Context

v0.1.0 (#42, #43) widened the release matrix (release.yml) to 8 targets, but the PR CI matrix (build.yml) still compiles only the original 5 (linux/{amd64,arm64}, darwin/{amd64,arm64}, windows/amd64). So a future change that breaks linux/386, linux/arm, or windows/arm64 won't surface until a release is tagged.

Fix

Add linux/386, linux/arm (GOARM=6), and windows/arm64 to build.yml's build matrix so every PR cross-compiles all 8 release targets. Compile-only is enough (no need to run tests on every arch).

Acceptance criteria

• build.yml build matrix == release.yml matrix (8 targets).
• A PR shows Build (linux/386), Build (linux/arm), Build (windows/arm64) checks, all green.

[saadqbal]

Merged via #46 — build.yml now compile-checks all 8 release targets (linux amd64/arm64/386/arm, windows amd64/arm64, darwin amd64/arm64) on every PR.