fix(cpu): PPCBUG-123/124/125/126/161/162/566 XER TBC + lswi/stswi/lmw

Phase 6 batch 2 — XER TBC enabling + load/store-multiple cleanups.

- PPCBUG-123/124/161/566 (coupled): XER TBC field was unmodelled —
  `ctx.xer()` always returned 0 in bits 0-6, and `ctx.set_xer()`
  silently discarded any TBC writes. Result: `lswx` and `stswx` were
  permanent no-ops (the `while bytes_left > 0` loop never executed).
  Fix: add `pub xer_tbc: u8` to `PpcContext`; wire into `xer()` and
  `set_xer()`. Initialize to 0 in `PpcContext::new()`. lswx/stswx
  bodies are correct as-is once the infrastructure is wired.

- PPCBUG-125 lmw: PowerISA marks `lmw rT, D(rA)` invalid when rA is
  in [rT..31]; canary skips the write to rA to preserve the EA base.
  Now matches canary.

- PPCBUG-126/162 lswi/stswi: replaced `instr.rb()` with `instr.nb()`
  for the NB field. Both accessors return identical values today
  (bits 16-20), but the maintenance hazard from the misnomer is now
  removed. A future `rb()` type-system refactor would have broken
  lswi/stswi silently.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/xenia-cpu/src/context.rs

@@ -85,6 +85,10 @@ pub struct PpcContext {
     pub xer_ca: u8,
     pub xer_ov: u8,
     pub xer_so: u8,
+    /// XER[25:31] string-byte count (`TBC`). Read/written by `mtspr XER`,
+    /// consumed by `lswx`/`stswx`. Per PPCBUG-123/124/161: was previously
+    /// unmodelled, making `lswx`/`stswx` a permanent no-op.
+    pub xer_tbc: u8,
     // Altivec VSCR. Only bits 16 (NJ) and 31 (SAT) of word 3 are meaningful.
     pub vscr: Vec128,
     // VRSAVE (SPR 256). Bitmask of which VRs need saving across context switches.
@@ -157,6 +161,7 @@ impl PpcContext {
             xer_ca: 0,
             xer_ov: 0,
             xer_so: 0,
+            xer_tbc: 0,
             // VSCR starts with NJ bit set (denormals flushed) — matches canary
             // thread_state.cc initialization.
             vscr: Vec128::from_u32x4(0, 0, 0, VSCR_NJ_MASK),
@@ -240,7 +245,10 @@ impl PpcContext {
 
     /// Get the full XER register value.
     pub fn xer(&self) -> u32 {
-        ((self.xer_so as u32) << 31) | ((self.xer_ov as u32) << 30) | ((self.xer_ca as u32) << 29)
+        ((self.xer_so as u32) << 31)
+            | ((self.xer_ov as u32) << 30)
+            | ((self.xer_ca as u32) << 29)
+            | (self.xer_tbc as u32)  // PPCBUG-123/566: bits 0-6 (TBC).
     }
 
     /// Set XER from a full 32-bit value.
@@ -248,6 +256,7 @@ impl PpcContext {
         self.xer_so = ((val >> 31) & 1) as u8;
         self.xer_ov = ((val >> 30) & 1) as u8;
         self.xer_ca = ((val >> 29) & 1) as u8;
+        self.xer_tbc = (val & 0x7F) as u8;  // PPCBUG-124.
     }
 
     /// Read the VSCR SAT (sticky saturation) bit.