Add cross-platform CI and packaging

[martinbha]

Summary

Add the continuous-integration and packaging foundation for macOS and Windows.

Changes

• pin Rust 1.85.0 and make the existing macOS code compile at the declared minimum version
• run formatting, workspace checks, strict Clippy, and tests on macOS 15 arm64 and Windows 2025 x86_64
• build and upload versioned native ZIP artifacts from both platform jobs
• embed Windows version information, DPI-awareness metadata, and GUI-subsystem behavior
• add local versioned packaging scripts for macOS and Windows
• document build prerequisites, artifact naming, supported architectures, and deferred signing/installer work

Validation

• cargo fmt --all -- --check
• cargo check --workspace --all-targets
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-targets (113 passed)
• RUSTDOCFLAGS=-D warnings cargo doc --workspace --no-deps
• cargo build --workspace --release
• Windows GNU cross-target check and strict Clippy
• workflow validation with actionlint
• macOS app packaging, plist validation, and ZIP integrity check
• Windows manifest XML validation
• dependency audit (no vulnerability failures; existing transitive GTK warnings remain)

Closes #9

[martinbha]

Review summary

Reviewed: the complete 12-file diff for Rust toolchain pinning, macOS and Windows CI, native packaging scripts, Windows executable resources, and build/release documentation.

Findings and fixes

No actionable findings were found. The declared Rust 1.85 minimum, native platform commands, packaging outputs, workflow permissions, pinned action revisions, and deferred distribution scope are internally consistent.

Validation

• Rust 1.85.0 formatting, workspace check, strict Clippy, 113 tests, warning-free documentation, and release build: passed
• Windows GNU cross-target check and strict Clippy: passed
• macOS bundle/plist/archive validation and workflow action lint: passed
• Native macOS 15 arm64 CI, package, and artifact upload: passed
• Native Windows 2025 x86_64 CI, package, and artifact upload: passed
• Downloaded Windows artifact identified as an x86-64 Windows GUI executable with embedded resources
• Dependency audit: no vulnerability failures; existing transitive GTK warnings remain
• Required PR checks: passed

Result

No remaining known actionable findings.