Add JSON test scenarios, patch/bank scripting, and performance stats (Phases 2-4)

- TestScenario: parses a JSON file describing sample rate/block size/
  duration, an input signal config (silence/sine/noise/impulse), and a
  timed sequence of events (noteOn/noteOff/controller/param/loadPatch/
  loadBank/savePatch/saveBank), sorted by sample position. Relative
  file paths inside the JSON resolve against the scenario file's own
  directory, not the process cwd, so scenario files are portable.
  Nothing here is JaySynth-specific: an event type a given plugin
  doesn't care about (e.g. MIDI sent to a plugin with no MIDI input)
  is simply inert, since AudioProcessor/MidiBuffer already tolerate
  that generically.
- main.cpp: unified the ad-hoc CLI flags (--note/--param/--input/etc.)
  and --scenario JSON files through one render loop, by synthesizing
  a TestScenario from the CLI flags when no JSON file is given. Added
  --bank/--savePatch/--saveBank for the ad-hoc path (loadPatch/wav
  already existed). Non-MIDI events (param/patch/bank) scheduled
  inside a block apply at the start of that block; MIDI events keep
  full per-sample timing via MidiBuffer's own offset parameter.
- PerformanceStats: wraps each processBlock call with a high-resolution
  timer, reports min/mean/max render time and a real-time factor
  (audio seconds rendered / wall-clock seconds), plus optional
  per-block CSV export for tracking regressions over time. Never
  paces the loop to real-time - the point is to run faster than that.

Verified end-to-end:
- Ad-hoc CLI path against JaySynth.so still renders correctly and now
  reports performance (e.g. ~74x real-time on this machine).
- A JSON scenario driving JaySynth (timed noteOn/param/noteOff/
  savePatch) produces correct WAV output and a valid saved patch file.
- That saved patch round-trips back in via --patch and renders
  cleanly (proves the save/load path is consistent, not just
  one-directional).
- A JSON scenario driving a real third-party effect (ZamComp-vst.so,
  2 in/1 out - a genuinely asymmetric channel count, not just "0 in"
  or "stereo") with generated pink noise and a param event produces
  correct mono output at >1000x real-time.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011dhtwRLARk4eiPngcQykLJ

tools/testhost/src/config.mk

@@ -4,3 +4,4 @@ CXX_SRCS := main.cpp
 CXX_SRCS += PluginHost.cpp
 CXX_SRCS += AudioInputSource.cpp
 CXX_SRCS += WavRecorder.cpp
+CXX_SRCS += TestScenario.cpp