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/tests/golden/sylpheed_n2m.json

@@ -0,0 +1,16 @@
+{
+  "path": "/home/fabi/RE Project Sylpheed/Project Sylpheed - Arc of Deception (USA, Europe) (En,Ja).iso",
+  "instructions": 2000000,
+  "imports": 5634,
+  "unimpl": 0,
+  "packets": 0,
+  "draws": 0,
+  "swaps": 0,
+  "resolves": 0,
+  "unique_render_targets": 0,
+  "shader_blobs_live": 0,
+  "interrupts_delivered": 0,
+  "interrupts_dropped": 13,
+  "texture_cache_entries": 0,
+  "texture_decodes": 0
+}