feat(pj_base): pj.data_processors.v1 — data-only SDK service for plugin-created transforms

[facontidavide]

What

Adds the pj.data_processors.v1 host service — a data-only C ABI that lets a toolbox plugin create catalog-resident Transform nodes in the host. Only strings cross the boundary (script source + input/output names + params JSON + an id); the host owns all execution.

This is M4.1 of the PJ4 Data Processors plan (the SDK half). The PJ4 host bridge (M4.2) that turns these calls into live DerivedEngine nodes lands separately in the application repo.

Why this shape

• Plugin creates, host executes. Because the host owns the script text + VM, a transform survives plugin unload and session reload — the originating plugin is needed only for re-editing.
• No C++ across the DSO boundary. Nothing executable crosses; this deliberately avoids the C++-vtable / library_owner UAF that sank the earlier filter-registry attempt.
• The native door is WASM, not a DSO kernel. The payload is binary-safe (PJ_string_view_t {data,size}), so a future host-owned WASM/Python backend ships as bytes through this same surface — purely additive, survives unload, sandboxed — rather than a native kernel vtable that would dangle on unload.
• Transforms only. outputs is required non-empty (named catalog topics); host-internal per-curve Filters are not created through this ABI but share the same DataProcessor substrate.

Changes

• PJ_data_processors_host_vtable_t (create / remove / list / config), protocol_version 1, struct_size-gated, ABI-appendable; all slots [main-thread], PJ_NOEXCEPT.
• sdk::DataProcessorsHostView C++ sugar + DataProcessorsHostService trait.
• View now rejects an empty output list before reaching the vtable.
• Doc-comments: binary-safety, per-plugin isolation, transactional create, the WASM forward-compat door.

Versioning

Additive new service → Conan MINOR 0.8.0 → 0.9.0; abi/baseline.abi untouched (additions-only).

Tests

Fake-host gtest suite, 8 cases (6 baseline + CreateRejectsEmptyOutputs, BinarySafePayloadRoundTrips). Full SDK build + 47/47 tests green; pre-commit (clang-format v22.1.0) clean.

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