Prototype a framebuffer text console #178

Summary

Provide a basic text renderer for a supported framebuffer pixel format.

Scope

• Define the subsystem contract, ownership, initialization order, and failure behavior.
• Integrate with current Rustalis interfaces instead of creating a parallel implementation.
• Keep the kernel Rust-first, no_std, and x86_64-compatible; document architecture-specific unsafe code.

Acceptance criteria

• Normal behavior and invalid/boundary inputs are covered by tests.
• Host tests cover hardware-independent logic; QEMU tests cover hardware paths where practical.
• Documentation distinguishes implemented behavior from planned work.
• Relevant formatting, workspace checks, and tests pass, or blockers are documented.
• Do not claim verification that has not actually been run.