Add regression suite testing JaySynth's TODO.md findings via testhost

tools/testhost/tests/run_tests.sh dynamically exercises the tracked
hardening findings using testhost itself - no GUI, sub-second wall
time:

- oversized_blocksize (Critical #5): --blockSize 16384 under a
  timeout, catching either the overflow or the deadlock the original
  bug could cause.
- nrpn_out_of_range/nrpn_in_range (Critical #1): JSON scenarios
  sending the 5-message MIDI NRPN CC sequence, once targeting the
  max 14-bit ID (16383) and once a legitimate in-range ID (500), so
  a regression that made the bounds check too aggressive would also
  show up.
- patch_save_load_roundtrip / bank_save_load_roundtrip: save a
  distinctive continuous parameter value, reload in a fresh process,
  check it round-trips within tolerance - covers the
  setCurrentProgramStateInformation no-op bug and the
  patchDecodeXml/patchImportXml fixes.
- stability_sweep: loads every bundled .fxp under extras/sounds/ and
  renders a held note through each, as a broad crash/NaN net.

Added --printParams to main.cpp (prints every parameter's index and
value) to make the round-trip tests possible - this was also already
a noted nice-to-have in testhost/TODO.md.

While building the oversized_blocksize test, found and fixed (on the
hardening branch, commit d713ca5) a second, previously-unknown bug:
VCF_CalcCoeff_LPF/_HPF/_BPF advance the coefficient buffer pointer
once per (sample, filter-section) pair, but the buffer was only ever
allocated for bufsize samples, not bufsize*sections - a 4th-order
filter overflowed it for any block between 4097 and 8192 samples.
Found via bisecting the crashing block size then confirming with an
ASan build; see tests/README.md's debugging-notes section for exactly
how, including the ASan symbolizer hang encountered along the way and
the workaround.

Also discovered (documented in tools/testhost/TODO.md's Investigate
section, not fixed - out of scope): JaySynth's compiled VST2 wrapper
never advertises chunk support to the host, so real .fxp/.fxb sample
files have no effect when loaded through the generic AudioProcessor
state API. The round-trip tests work around this by using testhost's
own save output as the fixture rather than an external sample file.

All 10 checks pass against the current hardening-branch build.

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

tools/testhost/README.md

@@ -141,6 +141,14 @@ the same call chain, so a fix to one doesn't automatically test the other.
   into an equivalent in-memory `TestScenario` before rendering starts, so
   there's only one loop to maintain).
 
+## Regression suite
+
+`tests/run_tests.sh` uses this tool to dynamically test the findings
+tracked in the repo-root `TODO.md` - see `tests/README.md` for what each
+test covers (and its limits). It's also how a real, previously-unknown VCF
+buffer-sizing bug got found and fixed (see that file and the repo-root
+`TODO.md`'s Critical section).
+
 ## Known limitations
 
 See `TODO.md` in this directory.