feat(sdk): canonical-object pipeline — SchemaHandler table, BufferAnchor, push_message_v2 ABI

[pabloinigoblasco]

Summary

A coherent set of changes across pj_base and pj_plugins that establishes the canonical-object pipeline — the contract by which MessageParser plugins produce decoded scene primitives (images, point clouds, …) and the host routes them through the ObjectStore.

This is a design sketch posted as a draft for review. It compiles cleanly with the companion parser_ros / runtime work and unit tests pass, but the end-to-end flow against real data sources is still pending validation.

What this adds

Canonical-object SDK (pj_base/sdk/canonical_object.hpp)

• BufferAnchor + PayloadView: zero-copy payload sharing — non-owning Span<const uint8_t> plus a shared_ptr<const void> that keeps the underlying bytes alive.
• sdk::Image, CompressedImage, PointCloud built around view + anchor; parsers return them without copying source bytes.
• CanonicalObjectKind enum + SchemaClassification.
• PixelFormat covers RGB/RGBA, BGR/BGRA, Mono8/Mono16.

MessageParser plugin base

• SchemaHandler table: per-schema registration with parse_scalars and parse_object callables. The base's classifySchema / parseScalars / parseObject are now table lookups. Plugins call registerSchemaHandler(...) in their constructor or bindSchema to declare what they know.
• parse() is no longer pure virtual: default implementation routes through parseScalars + writeHost.appendRecord, so plugins that register all their schemas inherit parse() for free.

C ABI extension

• PJ_payload_t / PJ_payload_anchor_t / PJ_payload_fetcher_t cross-ABI types: idempotent byte-fetcher with a release callback.
• New push_message_v2 tail slot on PJ_data_source_runtime_host_vtable_t: the DataSource hands the host an idempotent fetcher; the host applies the active ObjectIngestPolicy (kPureLazy / kLazyObjectsEagerScalars / kEager) to decide when (and whether) to invoke it.
• abi_layout_sentinels_test vtable size updated deliberately: 80 → 104 (MIN_VTABLE_SIZE pinned at the v4.0 baseline of 80).

SDK C++ helpers

• DataSourceRuntimeHostView::pushMessage template wraps a C++ closure (returning PayloadView or vector<uint8_t>) into a PJ_payload_fetcher_t and delegates to push_message_v2. Returns an explicit error when the host doesn't expose the slot — no silent fallback to the legacy raw-message path.
• ObjectIngestPolicy + ObjectIngestPolicyResolver with hierarchical override cascade: topic > data_source > kind > default.
• MessageParserHandle::classifySchema wrapper for the tail-slot call.

Tests

• object_ingest_policy_test: cascade rules at all four levels + last-write-wins.
• push_message_v2_test: mock host exercising the template's fetcher wrap (vector + PayloadView shapes), idempotency under repeated fetch, ctx lifetime via shared_ptr canary, anchor propagation past fetcher release, and the explicit error when the host predates the slot.

Test plan

• Build clean (RelWithDebInfo).
• Unit tests pass (object_ingest_policy_test, push_message_v2_test, abi_layout_sentinels_test).
• End-to-end with the companion parser_ros / runtime-host work — pending.

Companion work

The parser-side (parser_ros), the file source (mcap_source) and the runtime host (in the consumer app) ship as separate PRs to keep review surfaces focused.