feat(cli): add llamadart doctor diagnostics commands

[leehack]

Feature description

Add first-party diagnostics commands for users to inspect their local llamadart runtime setup before opening support issues.

Motivation

Recent backend, native-asset, Windows DLL, WebGPU, and load-time tuning work made runtime behavior more capable but also harder for users to self-diagnose. A doctor flow would turn common environment failures into actionable output.

Proposed scope

• Add a llamadart doctor command, or an equivalent package-supported diagnostics entrypoint.
• Report platform, OS, architecture, Dart/Flutter version, package version, selected backend, available emitted runtime libraries, and relevant environment variables.
• Detect common native failures: missing Linux runtime dependencies, Windows DLL resolution problems, backend bundle mismatch, and CPU/GPU backend selection surprises.
• Detect common WebGPU failures: browser support, secure-context requirement, adapter/features/limits, bridge asset version, and fallback behavior.
• Add optional machine-readable JSON output for issue reports and automated checks.
• Consider sibling commands such as llamadart backends and a bounded llamadart benchmark smoke check.

Acceptance criteria

• Diagnostics work without requiring a model when possible.
• Output includes clear remediation hints, not only raw environment data.
• Public docs include examples for native, Flutter desktop/mobile, and Flutter Web.
• Tests cover command formatting and at least the pure Dart capability-detection logic.