xenia-app: observability subsystem, --parallel runtime, stress harness

observability.rs installs the tracing subscriber stack (env-filter +
JSON file appender + chrome trace + error layer) and the metrics
recorder shared by the workspace. main.rs grows the new CLI surface:
--parallel, --reservations-table, --trace-handles, --analyze=
{rust,sql,both}, xenia dis --json, --ui, plus the wiring that runs
the CPU through the new scheduler, drives the GPU's threaded backend,
and surfaces the framebuffer + HUD via xenia-ui.

Add tests/parallel_stress.rs (#[ignore]-gated long form, short form
runs 20×@5M) and tests/golden/sylpheed_n2m.json — the digest the
lockstep/parallel combos compare against.

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

crates/xenia-app/Cargo.toml

@@ -20,9 +20,21 @@ xenia-apu = { workspace = true }
 xenia-hid = { workspace = true }
 xenia-debugger = { workspace = true }
 xenia-analysis = { workspace = true }
+xenia-ui = { workspace = true }
+winit = { workspace = true }
 tracing = { workspace = true }
 tracing-subscriber = { workspace = true }
+tracing-appender = { workspace = true }
+tracing-chrome = { workspace = true }
+tracing-error = { workspace = true }
+metrics = { workspace = true }
+metrics-util = { workspace = true }
+pprof = { workspace = true, optional = true }
 anyhow = { workspace = true }
 clap = { version = "4", features = ["derive"] }
 serde = { workspace = true }
 serde_json = { workspace = true }
+
+[features]
+default = ["profiling"]
+profiling = ["dep:pprof"]