Added VST SDK support.

docs/STEINBERG.md

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+# Steinberg SDK integration (Phase 106)
+
+The VST3 host uses Steinberg's pinned generated C ABI in the C99 core. Optional
+SDK examples now also build against the full, separately supplied official C++
+SDK and run Steinberg's validator. This adds an independent interoperability
+check without adding a C++ runtime or network dependency to libxtc.
+
+## Working integration
+
+- Linux `.vst3` bundles resolve to
+  `Contents/<architecture>-linux/<bundle-stem>.so`. Direct `.so` paths still
+  work. The original chosen path remains the saved identity reference. This
+  supports conventional bundles, not arbitrary renamed modules or Windows DLLs.
+- VST3 processing receives sample rate, absolute sample position, tempo and
+  musical position (`samples / rate * BPM / 60`). Position follows the existing
+  effect/instrument `_at` APIs, including seek. Tempo is the prepared plugin
+  spec's milli-BPM value; reprepare when it changes. Processing is marked playing
+  because this API represents active rendering, including offline rendering.
+  Loop, recording, wall-clock, time signature and continuous-time flags are not
+  asserted: the current plugin API does not carry that information. Positions
+  above `INT64_MAX` reject before processing, matching VST3's signed sample clock.
+- Original SDK-built examples expose a mono half-gain effect and a mono,
+  16-channel sine instrument. The instrument handles note-on/off at their exact
+  block offsets; the host translates all-notes-off to held-note releases.
+- The C render example demonstrates the actual xtc plugin facade and WAV writer,
+  including no-clobber output, without requiring JACK or a GUI.
+
+See [the example instructions](../examples/steinberg/README.md) for SDK versions,
+build commands, validation and ASIO diagnostics.
+
+## ASIO and VST2 scope
+
+ASIO is native driver I/O, separate from VST plugin hosting. The optional Windows
+diagnostic enumerates drivers and inspects an explicitly selected driver's
+channels, sample types, current sample rate, buffer bounds and reported latency.
+It does not create/start audio buffers and is **not an xtc ASIO backend**.
+The current xtc engine/backend still targets Linux/POSIX and JACK/PipeWire.
+Windows portability, a C API platform bridge and native-driver qualification
+are remaining Phase 106 work, not simulated by Linux tests.
+
+VST2 support retains the existing original minimal ABI declarations. VST3's SDK
+does not include the discontinued VST2 SDK. No VST2 SDK or user plugin is copied,
+fetched or licensed by these build targets.
+
+## Remaining work
+
+The application VST3 adapter still accepts one mono input (or no audio input for
+instruments), one mono output, float32 and zero reported latency. Its two mono
+instances are not a native stereo plugin instance. It uses plugin defaults;
+controller parameters, opaque-state persistence and custom editor windows remain
+unimplemented in xtc. The SDK examples include an edit-controller class for
+standard SDK-host compatibility; xtc does not instantiate it yet. The examples
+have no editable parameters or opaque state, so they do not prove those paths.
+No VST extension support or general third-party compatibility is inferred from
+the validator passing these examples.
+
+The complete acceptance checklist is in [Phase 106](ROADMAP.md#phase-106--steinberg-vst-sdk-and-asio-integration).