CONTEXT.md: runtime device detection milestone + QEMU hotplug testability notes

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@@ -294,6 +294,42 @@ Workspace of four crates, all built for the `x86_64-unknown-uefi` target:
   temporary 140-line boot loop (128 kept / 40 dropped, seq 40..167,
   oldest live line correct — loop reverted after the test).
 
+✅ **Runtime device detection (hotplug rescan)** — the boot discovery
+  pipeline is now re-runnable and diffed against live hardware:
+
+- `ahci`: stores ABAR/port bitmap; `poll_dirty()` is a cheap per-port
+    `PxSSTS.DET == 3` probe (safe on every prompt), `rescan()` starts
+    newly-present ports (full `start_port` bring-up) and tombstones
+    slots of removed ones.  **Slots never shift** — indices stay stable
+    so VFS disk handles and `ACTIVE` selection keep working; `select()`
+    rejects tombstones.
+- `fat`: `Volume` gains a `mounted` flag; `rescan()` unmounts volumes
+    whose disk left and mounts every disk without a mounted volume
+    (which also re-tries mounts that failed at boot, e.g. a non-FAT
+    disk replaced by a FAT one).  UID dedup ignores tombstones, so a
+    re-plugged disk keeps its `/disk/<UID>` name.
+- `fs`: the init mount loop is factored into `rescan_disks()`, which
+    lazily creates `/disk`, ensures a mount node per mounted volume and
+    tombstones nodes of unmounted ones — `cat /disk/<gone>/…` fails
+    cleanly with "no such file".
+- shell: `rescan` command plus an automatic deep rescan before each
+    prompt whenever `poll_dirty()` sees a change; a one-line summary
+    reports added/removed counts.  All rescan events flow through
+    `serial_println!` so they land in klog (`ahci: port N up`, `fat:
+    mounted …`) — the boot log doubles as a device log.
+
+  **QEMU testability reality (verified)**: PCIe ADD works end-to-end
+  (`-device pcie-root-port,id=rp1` at boot, monitor `device_add
+  e1000e,bus=rp1` → the kernel's `pci` re-scan sees it; q35 root bus
+  itself rejects hotplug, root ports are required).  Disk hotplug is
+  **not** QEMU-modelable: the ich9-ahci's IDE buses refuse
+  `device_add ide-hd` ("Bus 'ide.N' does not support hotplugging") and
+  `drive_add if=ide` ("Can't hot-add drive to type 1"); PCIe
+  `device_del` is a guest-cooperative protocol (the device stays in
+  config space until the guest powers off the slot), so removal is
+  also untestable in QEMU.  The disk arrival/removal path (PxSSTS
+  based) is verified on real hardware instead.
+
 ## Known issues / perf notes
 
 - **Scroll cost** — scrolling still shifts the whole framebuffer (a