R0-1: introduce protocol-neutral ProcessBlockView without changing Single behavior

[masarray]

Refs #63

Base fixed point: 060197707a372adce55db63434e7ab709a3d8262.

Mandatory pre-mutation proof

Before any production process mutation, the real-VST3 characterization harness exercised the unchanged Single Vst3Engine::process(AudioSlot&) seam using deterministic fixed-mono and fixed-stereo VST3 fixtures built from the repository's pinned Steinberg SDK.

That gate passed on source head 819e394949184ed64d86901b0cf3e13a3306c6ea in R0-1 Process Seam Characterization #8, run 33135381993.

The proof locks:

• direct mono and stereo processing;
• stereo-host -> mono-plugin and mono-host -> stereo-plugin adaptation;
• invalid block rejection;
• real IAudioProcessor::process() failure propagation;
• projectTimeSamples continuity;
• failed blocks do not advance sample position.

R0-1 implementation

Only after that gate passed:

• added protocol-neutral, non-owning ProcessBlockView;
• added Vst3Engine::process(const ProcessBlockView&) as the deep processing entry;
• kept process(AudioSlot&) as a stack-only Single transport adapter;
• added deterministic parity coverage comparing independent AudioSlot and ProcessBlockView engine instances;
• preserved the existing process-context source contract by following block.frames at the neutral seam;
• extended Compatibility Test Build path filters so this host seam cannot bypass the mandatory OBS compatibility/package gate.

Single AudioSlot/shared-memory layout and protocol version are unchanged. No Rack protocol/helper, HostedPlugin extraction, multi-plugin runtime, Rack OBS filter, production Rack editor, scanner redesign, or Single protocol redesign is included.

Real-machine P0 found and fixed before merge

The first exact-candidate smoke on OBS Studio 32.1.2 exposed an OBS source-registration ABI mismatch: the module built against a newer SDK forwarded sizeof(obs_source_info)=424 while that runtime supported 408 bytes, so the DLL loaded but obs_safe_vst3_filter failed to register.

The narrow compatibility fix is commit dc3a52bdb8b6a15026f39c46130b42f0acee5c6a: obs_register_source() is redirected to obs_register_source_s() with the ABI prefix ending at obs_source_info::save, the last source-info field this module actually populates. No DSP/process behavior, transport layout, Rack code, or installer behavior changed.

The repaired candidate was then reinstalled on the same real OBS 32.1.2 machine and VST 3.x Plug-in is visible again in Add Filter. Source registration visibility is therefore PASS on the repaired exact candidate.

Frozen exact candidate

dc3a52bdb8b6a15026f39c46130b42f0acee5c6a

Git tree: 956e8b3a5720e8242ea2993263521a2bca7d9c51.

main remains at 060197707a372adce55db63434e7ab709a3d8262, so there is no base drift.

The PR merge ref 95734c03691c2d368e59c714c357d91faedb88aa resolves to the same Git tree 956e8b3a5720e8242ea2993263521a2bca7d9c51.

Automated exact-head qualification — PASS

• CI #375 / run 33161584578: SUCCESS on dc3a52bdb8b6a15026f39c46130b42f0acee5c6a;
  • protocol-linux SUCCESS;
  • helper-windows SUCCESS;
  • obs-module-windows SUCCESS against current OBS SDK;
  • installer-windows SUCCESS.
• R0-1 Process Seam Characterization S2.4 final merge checklist #17 / run 33161584526: SUCCESS.
• Compatibility Test Build #162 / run 33161584519: SUCCESS;
  • Windows regression tests + scanner smoke;
  • official OBS 29.1.3 loader/ABI-floor probe;
  • package build + PE dependency inspection;
  • portable validation;
  • canonical OBS-root installer smoke;
  • artifact upload.

Compatibility #162 artifact:

• artifact ID 9682202575;
• artifact name OBS-Safe-VST3-Host-v0.4.1-stabilize-test5-windows-x64;
• artifact digest sha256:ee5802f1d18ee61eabb88057973a30830bffb0214c46e03b944e1eb89ddd72e5;
• exact workflow head dc3a52bdb8b6a15026f39c46130b42f0acee5c6a.

Remaining merge gate

Per ADR-0004 item 11 / issue #63, merge remains gated on completing the representative exact-candidate real-machine Single smoke on dc3a52bdb8b6a15026f39c46130b42f0acee5c6a:

1. normal commercial-VST3 audio;
2. helper-owned vendor editor open/hide/reopen;
3. exact-state continuity through helper recreation;
4. dry fail-open behavior.

Filter registration visibility on OBS 32.1.2 is already PASS after the ABI fix, but that alone does not satisfy the four-point smoke above.

Until those four observations are recorded, this PR remains open, issue #63 remains open, and R0-2 must not start.