rsnix: runtime device detection — rescan for hotplugged/removed disks

Phase 1: make the boot discovery pipeline re-runnable and diff it
against live hardware state.

- ahci: store ABAR/NP/port bitmap; poll_dirty() (cheap PxSSTS.DET check
  per port) and rescan() which starts newly-present ports and tombstones
  the slots of removed ones.  Slots never shift, so disk indices stay
  valid.  select() now rejects tombstoned slots.
- fat: Volume gets a mounted flag; rescan() unmounts volumes whose disk
  left and mounts every disk without a mounted volume (re-trying failed
  boot mounts).  UID dedup ignores tombstones so a re-plugged disk keeps
  its /disk/<UID> name.
- fs: init's mount loop factored into rescan_disks(), which lazily
  creates /disk, ensures a mount node per mounted volume and tombstones
  nodes of unmounted ones.
- shell: 'rescan' command + automatic deep rescan before each prompt
  when ahci::poll_dirty() sees a port-presence change.

Rescan events flow through serial_println so they land in klog too.
Removal (PxSSTS) is a real-hardware path: QEMU q35 rejects disk hotplug
on the ich9-ahci IDE buses, and PCIe device_del is a guest-cooperative
protocol this kernel does not implement.

1 files changed, 35 insertions, 0 deletions

diff --git a/shell/src/lib.rs b/shell/src/lib.rs
index ddd8688..28eb737 100644
--- a/shell/src/lib.rs
+++ b/shell/src/lib.rs
@@ -45,6 +45,12 @@ pub fn run() -> ! {
         // Push any deferred scrolls so the prompt renders on a current
         // framebuffer (e.g. after a streaming command's batch of output).
         rsnix::vt::sync_scroll();
+        // Cheap hotplug probe: if any AHCI port's device presence changed
+        // since the last scan, re-run discovery (mounts new volumes,
+        // unmounts gone ones) before showing the prompt.
+        if rsnix::ahci::poll_dirty() {
+            report_rescan(rescan_all());
+        }
         // Print prompt
         term_print("> ");
 
@@ -681,6 +687,7 @@ write `{} <path> TIL <marker>`",
         "stress" => cmd_stress(&parsed),
         "date" => cmd_date(&parsed),
         "pci" => cmd_pci(&parsed),
+        "rescan" => cmd_rescan(&parsed),
         "ls" => cmd_ls(&parsed),
         "cat" => cmd_cat(&parsed),
         "klog" => cmd_klog(&parsed),
@@ -710,6 +717,7 @@ fn cmd_help(_p: &Parsed) {
   stress   — stress-test heap/page allocators
   date     — show current date/time (RTC)
   pci      — list PCI devices
+  rescan   — re-scan for hotplugged/removed disks (also runs automatically)
   ls       — list a directory
   cat      — print files (cat USE a TO b copies a→b; cat TIL END is a heredoc)
   klog     — kernel boot log (cat /dev/klog; WRITING <dst> persists it)
@@ -1527,6 +1535,33 @@ fn cmd_date(p: &Parsed) {
     pipe.end();
 }
 
+/// Deep device re-scan: AHCI ports → FAT volumes → VFS mount table.
+/// Returns (disks added, disks removed, vols mounted, vols unmounted,
+/// nodes added, nodes removed) so callers can report the result.
+fn rescan_all() -> (usize, usize, usize, usize, usize, usize) {
+    let (da, dr) = rsnix::ahci::rescan();
+    let (ma, mu) = rsnix::fat::rescan();
+    let (na, nr) = rsnix::fs::rescan_disks();
+    (da, dr, ma, mu, na, nr)
+}
+
+/// Print the one-line change summary; returns true if anything changed.
+fn report_rescan((da, dr, ma, mu, na, nr): (usize, usize, usize, usize, usize, usize)) -> bool {
+    term_println(&alloc::format!(
+        "rescan: {} disk(s) added, {} removed · {} volume(s) mounted, {} unmounted · {} node(s) added, {} removed",
+        da, dr, ma, mu, na, nr
+    ));
+    da + dr + ma + mu + na + nr > 0
+}
+
+fn cmd_rescan(_p: &Parsed) {
+    if report_rescan(rescan_all()) {
+        term_println("rescan done — /disk updated");
+    } else {
+        term_println("rescan done — nothing changed");
+    }
+}
+
 fn cmd_pci(p: &Parsed) {
     let devices = rsnix::pci::probe();
     let mut pipe = Pipeline::new(&p.filters, &p.output);