[2.BF] Synthetic silph::WorkerCtx spawn (round 18 — opt-in landing)

Adds infrastructure to synthesise the silph::WorkerCtx that AUDIT-058/059
identified as never reached by ours' static-init chain (real chain entry
sits in audit-059 round 9's wrong-vtable wedge at sub_82172BA0+0x1E8).
Ctx layout follows round 5's live hexdump from canary:

  +0x00   vtable = 0x8200A1E8
  +0x04   self
  +0x08   intrusive list head -> self
  +0x0C   init flag = 1
  +0x10   packed byte field
  +0x18   2x float ~1.0 (UI rates)
  +0x24   flag = 1
  +0x28..+0x30  3x foreign-arena pointers (left NULL — see below)
  +0x54..+0x84  4x X_KEVENT auto-reset, state=0
  +0x94..+0xC4  4x X_KEVENT manual-reset, state=1 (pre-signaled)
  +0x210..+0x250  4-entry intrusive work-ring, empty

Worker spawn mirrors AUDIT-048's audio-worker pattern in
xaudio_register_render_driver: per-worker allocate_thread_image +
state.scheduler.spawn with r3 = ctx_ptr. Trigger fires at the first
dat/* VFS open (ours' earliest is dat/files.tbl), which is when canary
runs the equivalent chain.

ROUND 18 OUTCOME — opt-in only:

With workers spawned Ready (XENIA_SILPH_SYNTH=1), boot CRASHES at
cycle ~5.5M with PC=0 on hw=1, just after worker_3 (entry 0x825065B8)
spawns. Per task constraints this is STOP-and-report: the ctx fields
+0x28/+0x2C/+0x30 (foreign heap pointers — canary's 0x30057018,
0xBCE25640, 0xBE568F00, distinct arenas per audit-059 round 7) are
left NULL, and the worker bodies plausibly dereference one of them.
Synthesising those is a fresh investigation (round 19+).

With workers spawned Suspended (XENIA_SILPH_SYNTH=suspend), boot
completes normally (11 spawns, VdSwap=1, KeSetEvent=2,
KeReleaseSemaphore=1 — matches default baseline). The ctx remains
materialised in guest memory at the logged VA for downstream probing.

Default (env var unset): no synth, no regression.

Files:
  crates/xenia-kernel/src/silph_synth.rs   (new, 225 LOC)
  crates/xenia-kernel/src/lib.rs           (+1 LOC, register module)
  crates/xenia-kernel/src/exports.rs       (+37 LOC, hook in open_vfs_file)
  crates/xenia-kernel/src/state.rs         (+18 LOC, 4 silph_synth_* fields)

Tests: cargo test --release --workspace = 765 pass / 0 fail.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/xenia-kernel/src/lib.rs

@@ -3,6 +3,7 @@ pub mod exports;
 pub mod interrupts;
 pub mod objects;
 pub mod path;
+pub mod silph_synth;
 pub mod state;
 pub mod thread;
 pub mod ui_bridge;