Clean up warnings, fix FAT32 multi-cluster dirs, prep for edition 2024

Warning cleanup (cargo check + clippy now 0 across the workspace):
- Migrate all 21 static_mut_refs sites to &raw mut accessors (fat, gop,
  ps2, vt, ahci, interrupts, memory) so edition 2024 won't deny them
- panic = "abort" profiles: bare cargo check/build now works without
  nightly/-Zbuild-std
- Wire TICKS in the timer ISR (it was never incremented, so the vt scroll
  batching trigger was dead code)
- Clippy: efi_main becomes unsafe extern "efiapi" with a # Safety doc,
  KeyBuf::new uses inline-const array repeat (no interior-mutable const),
  editor main loop is a while-let, PIT control byte regrouped
- [workspace.lints.clippy] allows result_unit_err + needless_range_loop
  (deliberate kernel style); large_enum_variant allowed on LineInput
- Drop now-stale static_mut_refs allow from the rsdos bin

FAT32 correctness fixes (both pre-existing, exposed by the SFN fix):
- Unique 8.3 short names: to_8_3() never disambiguated collisions, so
  files sharing a long-name prefix got identical SFNs and fsck reported
  duplicate directory entries.  Add dir_short_names() + unique_short_name()
  (~N tails, Windows-style) and force LFN when the SFN changes.
- Multi-cluster directories: scan_dir/dir_short_names stopped at the first
  0x00 slot, hiding every entry written into a chained cluster (writes
  failed with 'cannot write', duplicate SFNs accumulated); add_dir_entry
  now packs the cluster (0xE5 deleted-markers) before extending so other
  FAT readers (mtools, Windows) see the continuation too.

Other: delete dead fields (Disk.fis, Volume.fat_count), drop timer_irq/
keyboard_irq/rendered, refresh_and_exit is a documented unsafe fn, CONTEXT
.md corrected (stable toolchain, IRQ input, 4-crate layout), .gitignore
excludes .pi-subagents artifacts.

Validated: cargo check/clippy/fmt clean, make test-disk PASS on fresh and
reused images (incl. two directory-chain extensions and ~10 SFN tails),
editor round-trip smoke test PASS.

1 files changed, 7 insertions, 3 deletions

diff --git a/rsnix/src/ps2.rs b/rsnix/src/ps2.rs
index 4747813..35848b6 100644
--- a/rsnix/src/ps2.rs
+++ b/rsnix/src/ps2.rs
@@ -263,8 +263,12 @@ pub fn try_poll_key() -> Option<KeyEvent> {
 
 /// Process a single scancode byte. Returns Some(KeyEvent) when a visible
 /// event happens; returns None for modifier changes, releases, and prefixes.
+fn kbd() -> &'static mut KbdState {
+    unsafe { &mut *core::ptr::addr_of_mut!(KBD) }
+}
+
 fn process_byte(byte: u8) -> Option<KeyEvent> {
-    let state = unsafe { &mut KBD };
+    let state = kbd();
 
     if byte == 0xE0 {
         state.got_e0 = true;
@@ -368,7 +372,7 @@ fn process_e0(byte: u8) -> Option<KeyEvent> {
 }
 
 fn update_modifiers(scancode: u8, pressed: bool, e0: bool) {
-    let state = unsafe { &mut KBD };
+    let state = kbd();
     match (scancode, e0) {
         (0x2A, false) => set_mod(state, MOD_LSHIFT, pressed),
         (0x36, false) => set_mod(state, MOD_RSHIFT, pressed),
@@ -393,6 +397,6 @@ fn set_mod(state: &mut KbdState, bit: u8, pressed: bool) {
 
 /// Is either Ctrl key held?
 fn ctrl_held() -> bool {
-    let state = unsafe { &mut KBD };
+    let state = kbd();
     state.modifiers & (MOD_LCTRL | MOD_RCTRL) != 0
 }