M11–M12: IRQ-as-IPC and bus drivers; expand names tree-wide

Two driver-model milestones plus a tree-wide naming pass. Suite 35/35
(QEMU) + host tests green.

M11 — IRQ-as-IPC. A ring-3 driver now sleeps until its device interrupts
it. New src/kernel/irq.zig: per-GSI endpoint bindings, comptime per-vector
trampolines, dispatch = mask GSI -> LAPIC EOI -> notifyLocked, all under one
lock region. irq_bind/irq_ack syscalls, gated by the device claim like
mmio_map. interruptDispatch no longer EOIs — each handler owns its EOI,
because a level line must be masked before it is acknowledged (irq_ack is
the unmask). Bindings are keyed on the owning task and released on exit
(a shared endpoint's siblings survive). hpetd rewritten interrupt-driven.
Tests: hpet (rewritten, reads back the I/O APIC routing) and irqfree.

M12 — bus drivers. DeviceDesc gains a parent, making the device table a
tree. dev_register (device_register) lets a process publish children below
a device it claimed; the kernel enforces resource containment (a child's
resources must nest in its parent's), so a descriptor can't fabricate a
window over kernel RAM. Descriptor copied in via copyFromUser (physmap
walk — an unmapped user pointer fails the call instead of faulting the
kernel). Per-parent child cap bounds table exhaustion. sbin/busd.zig is a
worked bus driver. Test: bus.

Naming — per docs/coding-standards.md: non-acronym abbreviations spelled
out (message, descriptor, device_service, scheduler, runtime, physical,
interpreter, ...); acronyms kept (IPC, MMIO, DMA, HCD, ...); files are
kebab-case (ipc-synchronous.zig, device-service.zig, vfs-protocol.zig, ...).
Exceptions: POSIX/C ABI names and Zig idioms (init/len/ptr) kept. Module
collisions resolved by specific naming (config -> parameters, device.zig
alias -> device_model). AML op/Op disambiguated: op = opcode, Op =
operation; per-opcode parse handlers renamed opX -> parseX.

New driver docs: drivers.md, driver-model.md (bus/class/HCD shapes + the
proposed M13–M16 ABI), coding-standards.md.

lib/system-call.zig

@@ -0,0 +1,52 @@
+//! Raw `system_call` instruction wrappers for user space — one per arity.
+//!
+//! ABI: number in rax, arguments in rdi, rsi, rdx, r10, r8, r9, result in rax.
+//! The `system_call` instruction itself clobbers rcx (it holds the return rip) and
+//! r11 (the saved rflags); the kernel entry stub preserves everything else.
+//! Note argument #3 goes in **r10, not rcx** — rcx is unavailable across the
+//! instruction, so the kernel reads the 4th argument from r10.
+
+const danos = @import("danos");
+const SystemCall = danos.SystemCall;
+
+pub inline fn systemCall0(n: SystemCall) usize {
+    return asm volatile ("syscall"
+        : [ret] "={rax}" (-> usize),
+        : [n] "{rax}" (@intFromEnum(n)),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+}
+
+pub inline fn systemCall1(n: SystemCall, a0: usize) usize {
+    return asm volatile ("syscall"
+        : [ret] "={rax}" (-> usize),
+        : [n] "{rax}" (@intFromEnum(n)), [a0] "{rdi}" (a0),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+}
+
+pub inline fn systemCall2(n: SystemCall, a0: usize, a1: usize) usize {
+    return asm volatile ("syscall"
+        : [ret] "={rax}" (-> usize),
+        : [n] "{rax}" (@intFromEnum(n)), [a0] "{rdi}" (a0), [a1] "{rsi}" (a1),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+}
+
+pub inline fn systemCall3(n: SystemCall, a0: usize, a1: usize, a2: usize) usize {
+    return asm volatile ("syscall"
+        : [ret] "={rax}" (-> usize),
+        : [n] "{rax}" (@intFromEnum(n)), [a0] "{rdi}" (a0), [a1] "{rsi}" (a1), [a2] "{rdx}" (a2),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+}
+
+pub inline fn systemCall4(n: SystemCall, a0: usize, a1: usize, a2: usize, a3: usize) usize {
+    return asm volatile ("syscall"
+        : [ret] "={rax}" (-> usize),
+        : [n] "{rax}" (@intFromEnum(n)), [a0] "{rdi}" (a0), [a1] "{rsi}" (a1), [a2] "{rdx}" (a2), [a3] "{r10}" (a3),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+}
+
+pub inline fn systemCall5(n: SystemCall, a0: usize, a1: usize, a2: usize, a3: usize, a4: usize) usize {
+    return asm volatile ("syscall"
+        : [ret] "={rax}" (-> usize),
+        : [n] "{rax}" (@intFromEnum(n)), [a0] "{rdi}" (a0), [a1] "{rsi}" (a1), [a2] "{rdx}" (a2), [a3] "{r10}" (a3), [a4] "{r8}" (a4),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+}