[Phase A] Port args / file.read extraction tracing onto instrument-current

Bring the cvar-gated JSONL extraction tracer (event_log) forward from the
phase-a-tracing line so args/args_resolved + file.read events are
available alongside the GPU draw logging. Purely additive:

- src/xenia/kernel/event_log.{cc,h}: the tracer (auto-globbed into the
  kernel target). Tier 1 fills kernel.call args (raw r3..r10) +
  args_resolved.path under --phase_a_trace_args; Tier 2 resolves
  NtReadFile handle->path via the object table and emits file.read.
- cpu_flags: phase_a_event_log_path / _mem_writes / _trace_args /
  _hash_probe / kernel_emit_contention (all default-off).
- shim_utils.h: phase_a_bridge decl + EmitImportAndCallWithCtx/EmitReturn
  hooks in the active X::Trampoline, skipped when Enabled() is false.

Deliberately EXCLUDES the Tier-3 hash-probe HIR/emitter trap (the IPFB
name-hash was recovered statically, so the probe is moot). The
phase_a_hash_probe cvar is defined but inert.

Default-off => instrument-current behaviour byte-identical when unused.
Not yet compile-verified against instrument-current's toolchain.

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

src/xenia/cpu/cpu_flags.cc

@@ -57,3 +57,28 @@ DEFINE_bool(break_condition_truncate, true, "truncate value to 32-bits", "CPU");
 
 DEFINE_bool(break_on_debugbreak, true, "int3 on JITed __debugbreak requests.",
             "CPU");
+
+// Phase A — expansive extraction tracing (game-data RE); see kernel/event_log.h.
+// All default-off so instrument-current behaviour is unchanged when unused.
+DEFINE_string(phase_a_event_log_path, "",
+              "Phase A: write schema-v1 JSONL event log to this path. "
+              "Empty (default) = disabled.",
+              "Audit");
+DEFINE_bool(phase_a_event_log_mem_writes, false,
+            "Phase A: include mem.write events in the JSONL log. RESERVED — "
+            "not wired in this phase. Default false.",
+            "Audit");
+DEFINE_bool(phase_a_trace_args, false,
+            "Phase A extraction: populate the kernel.call args (raw r3..r10) and "
+            "args_resolved (file I/O path/offset/length/buffer, alloc size, "
+            "handle names) fields, and emit file.read events. 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. "
+              "Default empty (off).",
+              "Audit");
+DEFINE_bool(kernel_emit_contention, false,
+            "Phase D Stage 1: emit `contention.observed` events. Default false "
+            "(zero cost when disabled). Requires --phase_a_event_log_path.",
+            "Audit");