Instrument: file.read offset tracer + XMA-PARAM probe (voice RE)

Additive, cvar-gated instrumentation used to reverse-engineer Project
Sylpheed's cutscene-voice storage (movie -> continuous sound-stream cue
region). Default-off; no behaviour change.

- event_log.cc: MaybeEmitFileRead emits `file.read` events with the real
  NtReadFile ByteOffset (from r10), length, handle->path, and buffer VA, so a
  sound.pNN read offset can be mapped to a sound.pak TOC entry.
  Gate: --phase_a_fileio_only=true --phase_a_event_log_path=<file>.
- cpu_flags.cc: phase_a_fileio_only cvar (+ xma_param_probe).
- xma_context_new.cc: XMA-PARAM probe in XmaContextNew::Decode (the decoder the
  title actually uses) logging ctx/buffer/read-offset/channels/packets/byte_size
  /signature — to confirm which bytes get decoded for a given cutscene.

Used to produce /tmp/rt_fileio.jsonl (RT01A playthrough), which cracked the
movie->voice mapping now implemented in sylpheed-reborn. Next: run the same
file.read trace on an unbound hokyu (e.g. hokyu_LS_s03A) to resolve which
VOICE_D the game streams for the 13 manifest-unbound resupply cutscenes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/xenia/cpu/cpu_flags.cc

@@ -73,6 +73,12 @@ DEFINE_bool(phase_a_trace_args, false,
             "args_resolved (file I/O path/offset/length/buffer, alloc size, "
             "handle names) fields, and emit file.read events. Default false.",
             "Audit");
+DEFINE_bool(phase_a_fileio_only, false,
+            "Phase A extraction (LIGHTWEIGHT): emit ONLY file opens (path) and "
+            "file.read events (path/offset/length) — no per-export import/kernel "
+            "call spam. Use to trace which sound.pak/movie bytes a title reads "
+            "(e.g. movie->voice mapping) without perturbing timing. Default false.",
+            "Audit");
 DEFINE_string(phase_a_hash_probe, "",
               "Phase A extraction: CSV of guest PCs — RESERVED (the HIR/emitter "
               "trap that drives it is not ported to instrument-current). Inert. "