docs: add ADAPTERS.md with adapter structure, rules, and scaffolding guide

docs/ADAPTERS.md

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+# Writing Adapters
+
+An adapter implements the `Platform` trait for a specific architecture. It is the only crate with `unsafe` code.
+
+## Scaffolding
+
+```bash
+scripts/new-adapter.sh <arch>
+```
+
+This generates `crates/adapter-<arch>/` with the standard structure. Then:
+
+1. Add `"crates/adapter-<arch>"` to workspace `members` in `Cargo.toml`
+2. Fill in `board.rs` with device addresses for the target board
+3. Add boot assembly (`global_asm!` in `main.rs` or a `.S` file)
+4. Update `linker.ld` for the target's entry address and memory layout
+5. Implement the TODOs in `hw/`, `mmu/`, `trap/`, and `platform.rs`
+
+## Structure
+
+Every adapter follows this layout:
+
+```
+crates/adapter-<arch>/
+├── Cargo.toml
+├── linker.ld
+└── src/
+    ├── main.rs          — boot entry, kmain
+    ├── board.rs         — all board/SoC constants in one place
+    ├── platform.rs      — impl Platform (the contract with the kernel)
+    ├── hw/              — one file per hardware device
+    │   ├── uart.rs      — serial output
+    │   ├── irq.rs       — interrupt controller (PLIC, GIC, APIC, ...)
+    │   └── timer.rs     — timer/clock
+    ├── mmu/             — memory management
+    │   ├── page_table.rs — architecture-specific page table format
+    │   └── allocator.rs  — boot-time page allocator
+    └── trap/            — trap/exception handling
+        └── mod.rs       — trap vector + Rust handler
+```
+
+## Rules
+
+- **`unsafe` is the absolute bare minimum.** Each `unsafe` block does exactly one hardware operation and is wrapped immediately in a safe function. You should be able to count every `unsafe` block on your fingers.
+- **`board.rs` is the single source of truth for addresses.** UART base, interrupt controller base, RAM start — all in one file. If someone ports to a different board with the same architecture, they change one file.
+- **The kernel never changes.** `Kernel<P: Platform>` is generic. Adding a new adapter means zero changes to `domain/`, `kernel/`, `types/`, or `platform/`.
+
+## Existing Adapters
+
+| Adapter | Architecture | Target Board | Status |
+|---------|-------------|-------------|--------|
+| `adapter-riscv` | RISC-V 64-bit | QEMU virt | Phase 2 complete — boots, MMU, traps, timer, IPC demo |
+
+## Platform Trait
+
+The adapter implements all methods of the `Platform` trait defined in `crates/platform/src/lib.rs`:
+
+```rust
+pub trait Platform {
+    fn configure_protection(&mut self, process: ProcessId, regions: &[MemoryRegion]);
+    fn context_switch(&mut self, from: &mut SavedContext, to: &SavedContext);
+    fn enable_user_mode(&mut self, entry: EntryPoint, stack: StackPointer) -> !;
+    fn set_timer(&mut self, duration: TimerDuration);
+    fn current_ticks(&self) -> Ticks;
+    fn mask_irq(&mut self, irq: IrqId);
+    fn unmask_irq(&mut self, irq: IrqId);
+    fn send_ipi(&mut self, target: CoreId);
+    fn core_id(&self) -> CoreId;
+    fn num_cores(&self) -> u8;
+    fn map_shared(&mut self, process: ProcessId, base: PhysicalAddress, size: PageCount, permissions: PermissionSet);
+}
+```
+
+## Boot Flow
+
+1. Assembly entry (`_start`): set stack pointer, call `kmain`
+2. `kmain`: zero BSS, init UART, init MMU, init traps, init timer
+3. Create `Kernel<AdapterPlatform>`, call `kernel.boot()`
+4. Start scheduling