feat: add macOS menu-bar status applet

[mm65x]

Summary

Implements the Phase 1 local macOS menu-bar client described in #26. The client consumes only the versioned tokemon status --json --offline interface and keeps parsing, pricing, aggregation, and discovery in the CLI.

Delivered

• Reports actual pricing availability in status JSON so unavailable cost is not mistaken for zero.
• Adds a macOS 13+ Swift package with an accessory MenuBarExtra executable.
• Invokes the configured executable directly with bounded stdout/stderr capture, a five-second timeout, and explicit launch, exit, encoding, and schema errors.
• Supports UTC today, ISO week, month, and all-time scopes.
• Persists executable path, metric, scope, and refresh interval preferences with safe defaults.
• Runs refreshes away from the UI, limits concurrent refreshes, and preserves the last successful result after failures.
• Shows tokens, cost, combined metrics, provider count, refresh time, empty/error/stale/unavailable states, manual refresh, dashboard launch, preferences, and quit.
• Adds contract, runner, query, preference, state, and UI-adjacent tests and a macOS Swift CI job.
• Documents local build, user-local install/removal, configuration, scope semantics, offline pricing, and troubleshooting.

Verification

• cargo fmt -- --check
• cargo test
• cargo clippy -- -W clippy::pedantic -A clippy::module_name_repetitions
• cargo build --release
• swift test (21 tests)
• swift build -c release
• GitHub CI is green for formatting, clippy, Linux/macOS/Windows Rust tests, and the macOS menu-bar package.

Out of scope

Signing, notarization, packaging/distribution, launch-at-login, notifications, provider toggles, accounts, credentials, telemetry, and network services remain follow-up work outside this local alpha.

Relates to #26.