[iterate-2T] VdSwap: route present through ring PM4_XE_SWAP, drop out-of-band swap interrupt

Make ours' VdSwap present path faithful to xenia-canary `VdSwap_entry`
(xboxkrnl_video.cc:518-548): write the reserved 64-dword ring slot with a
PM4_TYPE0 fetch-constant patch + PM4_TYPE3(PM4_XE_SWAP) + NOP padding, then
let the natural drain consume the swap packet in command-stream order. Remove
the synthetic CP swap-complete interrupt that `notify_xe_swap` raised
out-of-band.

Root found this session (the actual present-path bug): ours' `notify_xe_swap`
pushed an `InterruptSource::Swap` (→ INTERRUPT_SOURCE_CP) interrupt directly
from the VdSwap HLE, decoupled from the GPU command stream. When that interrupt
reached the graphics ISR `sub_824BE9A0` before D3D had armed its swap-callback
slot (`[gfx+10772]+16` still the `0xBADF00D` placeholder), the ISR took its
error path and hit the assert "ERR[D3D]: Unanticipated CPU_INTERRUPT. Sign of a
corrupt command buffer?" (`bl sub_824C5DF0; twi` at 0x824BE9DC) — 2x per run on
master. Canary's VdSwap raises NO interrupt; swap-complete CP interrupts come
only from in-stream PM4_INTERRUPT packets, which are naturally ordered after the
callback-arming Type-0 writes. Routing the swap through the ring packet matches
that ordering and eliminates the trap (2 -> 0).

Canary oracle confirmation (muted, audit_mem_watch + audit_jit_prolog_pc):
canary's early/loading loop is present-driven — swap counter [gfx+15160]
(0xBE56CA38) advances ~per-vblank from vblank 65 onward, reaching 0xD02 (3330)
in ~60s via 6184 CP source=1 interrupts, with VdSwap called only ONCE. So the
present interrupts are entirely in-stream, not from the VdSwap export.

This is a correctness/faithfulness fix; it does NOT cascade. draws stay 78 at
200M and 1B because the upstream gate persists: the game submits one render
batch then stalls (renderer sub_82506xxx 0x; 2nd title thread 0x821748F0 never
spawns). The per-frame loop sub_822F1AA8 runs ~1207 iterations on vsync but
clock B (swap count) only advances ~once, so the manager update sub_821741C8
fires once. That is the iterate-2Q/2F title-pipeline gate, not a present/
interrupt bug. swaps 3 -> 4 (the in-stream PM4_XE_SWAP now drains).

Deterministic in inline mode (n50m --gpu-inline --stable-digest regenerated
byte-identical twice; golden re-baselined: swaps 3 -> 4). cargo test --workspace
675 passing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/xenia-app/tests/golden/sylpheed_n50m.json

@@ -1,9 +1,9 @@
 {
-  "instructions": 50000013,
-  "imports": 451497,
+  "instructions": 50000001,
+  "imports": 451500,
   "unimpl": 0,
   "draws": 78,
-  "swaps": 3,
+  "swaps": 4,
   "unique_render_targets": 2,
   "shader_blobs_live": 3,
   "texture_cache_entries": 0