R0-2: extract deep HostedPlugin lifecycle/state seam

[masarray]

Closes #66

Parent: #56

Fixed-point base: bcddeb78e0b666d8ba2b4ae6d9ce355142181d5d

Scope is R0-2 only: introduce the protocol-neutral one-VST3 HostedPlugin lifecycle/state/process seam while preserving Single Host behavior and protocol. No Rack protocol/helper, multi-plugin runtime, Rack filter, GUI, presets, scanner redesign, MIDI, or R1-1 work.

TDD history begins with a deterministic real-VST3 stateful processor/controller fixture and source-contract test before production implementation. Final qualification must include R0-2 characterization, mandatory Single CI, Compatibility Test Build, Standards + Spec review, and exact-candidate real-machine Single smoke per ADR-0004 before merge.

[masarray]

R0-2 fixed-point Standards + Spec review

Reviewed exact source head: d9eb7a4af9d3e52ccbbe5587b8799d9e940fa583
Base: bcddeb78e0b666d8ba2b4ae6d9ce355142181d5d

Standards review

• Scope is R0-2 only. Diff contains HostedPlugin seam/types, deterministic R0-2 fixture/tests/workflow, and a behavior-neutral relocation of EngineParameter / EngineParameterUpdate definitions.
• No src/common/protocol.hpp change; Single protocol version/layout is unchanged.
• No src/host/main.cpp, OBS module, scanner, installer, recovery/state persistence, Rack helper/protocol/filter/UI/preset or multi-plugin runtime change.
• Existing Vst3Engine::process(AudioSlot&) remains the qualified Single compatibility adapter; R0-1 ProcessBlockView remains unchanged.
• Native editor implementation/ownership is not moved; HostedPlugin exposes only the controller accessor required by the accepted helper-owned editor boundary.

Spec review

• HostedPlugin is the new public protocol-neutral one-VST3 reuse seam. Its public header contains no AudioSlot, Single shared-memory layout, Rack topology/generation type, or protocol.hpp dependency; this is enforced by r0-2-hosted-plugin-source-contract.
• Lifecycle/state/process/parameter/latency/restart/editor-access responsibilities delegate to the already-qualified Vst3Engine implementation through PIMPL. No vendor lifecycle ordering is rewritten.
• The deterministic R0-2 fixture has a separated processor/controller, non-empty component and controller-private state, a parameter-driven observable processor state, and fixed latency. Characterization covers open/process, parameter catalog/change, component+controller state capture/restore and byte round-trip, latency restart, I/O restart, editor-controller accessor ownership, close/reopen and post-close process rejection.
• The first runtime characterization exposed the strict component-handler requirement; the harness was corrected to provide a lifetime-valid minimal handler. Production code did not need a relaxation.
• One heap allocation occurs only when constructing the control-plane HostedPlugin PIMPL; process(ProcessBlockView) adds no allocation or transport work.

Review outcome

No unresolved Standards/Spec code finding on d9eb7a4....

Qualification is still not merge-authorized until every exact-head automated gate is green and ADR-0004 item 11 real-machine Single smoke is recorded on the exact candidate.