GitHub Actions CI: build and test all targets

[enzoftware]

Description

Set up GitHub Actions CI to build and test all platform targets on every push and PR.

Tasks

• Workflow: Build & test silvestre-core on Linux, macOS, Windows
• Workflow: Build silvestre-cli on all three OSes
• Workflow: Build WASM with wasm-pack build
• Workflow: Build Flutter package (Android + iOS)
• Workflow: Run cargo clippy and cargo fmt --check
• Cache Cargo dependencies for faster builds
• Badge in README showing CI status

Acceptance Criteria

• CI runs on every push to main and on every PR
• All targets build successfully in CI
• Tests pass on all platforms
• Clippy produces no warnings
• Code formatting is enforced
• CI completes in a reasonable time (< 15 minutes)