Add cross-platform build target

[davebarnwell]

Summary

• add a make build-cross target for common macOS, Linux, and Windows binaries
• ignore the generated bin/ directory so cross-build artifacts do not show up in git status
• document the new build command in the README

Verification

• make help
• make build-cross

[Copilot]

Pull request overview

Adds a cross-compilation build target to produce common macOS/Linux/Windows binaries and keeps generated artifacts out of git status, with documentation updates for the new command.

Changes:

• Add make build-cross to build multiple GOOS/GOARCH binaries into bin/
• Ignore the generated bin/ build output directory in .gitignore
• Document make build-cross in the README

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
README.md	Documents the new make build-cross command alongside existing build/install commands.
Makefile	Introduces CROSS_PLATFORMS and a build-cross target that loops over OS/arch pairs and builds stripped binaries into bin/.
.gitignore	Adds an ignore rule for the bin/ directory so build artifacts don’t appear as untracked files.

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