Add BIOS and tiny kernel roadmap phases

Tutorial/phase-11-tiny-kernel/phase-11.md

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+# Phase 11 - Tiny Kernel + Command Shell
+
+## Context
+
+This phase builds an OS-shaped firmware payload before the project moves into traps,
+interrupts, privilege, DRAM, and Linux.
+
+## Goals
+
+- Load a separate GCC-built kernel ELF from the BIOS.
+- Provide a kernel-owned command shell.
+- Run tiny trusted ELF programs through a simple service table.
+
+## New Concepts
+
+- Kernel: central program that owns machine services. Before traps, this is still
+  trusted bare-machine firmware, not an isolated privileged OS.
+- Service table: function-pointer ABI for console I/O, exit, and simple utilities.
+- Bump allocator: simple allocator that hands out memory linearly.
+- Program table: small kernel data structure tracking loaded payloads.
+
+## How To Think About It
+
+The goal is a complete loop, not a Unix clone: compile a tiny program on the host,
+send it over UART, load it as ELF, run it on the CPU, and return to the shell.
+
+## Learning Tasks
+
+- Decide whether kernel console I/O calls the BIOS or drives UART directly.
+- Define the tiny-program ABI: entry registers, stack, service table, and return path.
+- Implement the smallest useful allocator.
+- Add shell commands that expose real machine state, not decorative output.
+
+## Pitfalls
+
+- Pretending trusted payloads are isolated before traps and privilege modes exist.
+- Growing the kernel into a second large project too early.
+- Hiding loader or ABI bugs behind ad hoc special cases.
+
+## Tooling And Testing
+
+- Keep kernel and user-payload linker scripts separate.
+- Build a tiny test suite of loaded programs: hello, echo, memory copy, return status.
+- Run the same payloads after Phase 12 trap handling lands to catch ABI regressions.
+
+## References
+
+- OSDev kernel structure notes: https://wiki.osdev.org/Kernel
+- RISC-V calling convention: https://github.com/riscv-non-isa/riscv-elf-psabi-doc
+- Embedded Artistry first-fit allocator background: https://embeddedartistry.com/blog/2017/02/15/implementing-malloc-first-fit-free-list/