Add runtime.time, drop demo drivers, harden TSC timekeeping

Time is a kernel concern in danos: the kernel owns the scheduling timer and
already exposes monotonic time via the clock/sleep/timer_bind syscalls, so a
userspace time service would be a redundant, slower path. This adds the generic
runtime.time module over those syscalls, retires the two demonstration drivers,
reorganizes the milestone docs, and makes the monotonic clock correct on Intel,
AMD, and inside any VM.

runtime.time (library/runtime/time.zig)
- Instant/Duration interface: now, sleep, spin, after, monotonicNanos, available
- a thin layer over system.clock/sleep/timerOnce; unit-tested arithmetic

Remove the demo drivers hpet and bus (a teaching example belongs in the docs,
not shipped in the tree)
- system/drivers/ now holds only real drivers: pci-bus, ps2-bus, usb-xhci-bus
- device-manager end-to-end test repointed to pci-bus (asserts on kernel state:
  the process table and the device tree, not a racy serial marker)
- device_register containment moved to a new in-kernel `containment` test
- the driver-model worked example moved inline into docs/drivers.md

Reorganize milestone docs into topic docs
- m17-m18 / m19-m20 / m21 plans dissolved into process-lifecycle, device-manager,
  discovery, and acpi docs; new docs/power.md and docs/timers.md; ~20 citations
  repointed; plan docs deleted

TSC reliability (apic.zig, smp.zig, cpu.zig, kernel.zig)
- check the invariant-TSC bit (CPUID 0x80000007 EDX[8]) on Intel and AMD
- cross-core "warp" check at SMP bring-up, pairwise BSP<->AP as each core comes up
- fall back to the HPET clocksource when the TSC is not invariant (a bare VM) or
  not synchronized (a warp), switched continuously so time never jumps
- boot log reports the outcome; new tsc-sync test exercises the TSC + warp path

Verified: zig build; zig build test; 60/60 QEMU cases (incl. new containment and
tsc-sync).

system/services/acpi/acpi.zig

@@ -1,5 +1,5 @@
 //! /system/services/acpi — the ACPI discovery service: the x86 firmware
-//! interpreter, moved out of ring 0 (docs/m19-m20-plan.md, M20). Claims the
+//! interpreter, moved out of ring 0 (docs/discovery.md). Claims the
 //! `acpi-tables` node the kernel publishes (the AML blobs, the broad io_port
 //! grant, a broad irq window, the SCI), and runs the **shared AML module** in
 //! ring 3 — the same parser and interpreter the kernel uses.
@@ -321,7 +321,7 @@ fn onSci() void {
 /// handler method (`_Lxx` level / `_Exx` edge), drain the Notify queue the
 /// method produced, and publish an event per notified device. Then clear the
 /// status bit. QEMU raises no GPEs on this config, so this path is exercised by
-/// host unit tests (docs/m21-plan.md decision 5); on real hardware it carries
+/// host unit tests (docs/acpi.md — ACPI events); on real hardware it carries
 /// battery/AC/lid. The embedded controller's `_Qxx` queries are out of scope.
 fn handleGpe() void {
     handleGpeBlock(gpe0_blk, gpe0_len, 0);
@@ -476,7 +476,7 @@ fn walkDevices(node: *aml.Node, interpreter: *aml.Interpreter) void {
 
         if (readHid(c, interpreter)) |hid| {
             // Skip PCI roots — pci-bus already reports PCI functions; ACPI adds
-            // only the non-PCI _HID devices (docs/m19-m20-plan.md M20.2). The two
+            // only the non-PCI _HID devices (docs/device-manager.md — matching). The two
             // roots are named through the shared registry, not bare _HID strings.
             const id = acpi_ids.HardwareId.fromHid(hid[0..7]);
             if (id != .pci_bus and id != .pci_express_root_bridge) {