docs(run): document the vix run architecture

[GaspardKirira]

Summary

Document the final architecture, supported inputs, dependency resolution,
planning, build integration, process lifecycle, watch mode, and compatibility
behavior.

Documentation sections

• supported input kinds;
• request and plan models;
• automatic dependency resolution;
• direct compilation;
• CMake fallback;
• project execution;
• executable artifact resolution;
• runtime arguments and environment;
• process lifecycle;
• sanitizers;
• watch mode;
• replay;
• exit codes;
• JSON events;
• migration from removed options.

Acceptance criteria

• Architecture documentation is available.
• CLI examples are included.
• Automatic dependency resolution is explained.
• Removed --auto-deps options are documented.
• Watch lifecycle is explained.
• Platform differences are documented.
• Documentation matches vix run --help.