…hor, push_message_v2 ABI

A coherent set of changes across pj_base and pj_plugins that establishes
the canonical-object pipeline.

Canonical-object SDK (pj_base/sdk/canonical_object.hpp):
- BufferAnchor + PayloadView for zero-copy payload sharing (Span<const
  uint8_t> view + shared_ptr<const void> anchor).
- sdk::Image, CompressedImage, PointCloud canonical types built around
  the view+anchor pattern so parsers can return them without copying
  the source bytes.
- CanonicalObjectKind enum + SchemaClassification descriptor.
- PixelFormat with kRGB888/kRGBA8888/kBGR888/kBGRA8888/kMono8/kMono16.

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

C ABI:
- 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.
- vtable size sentinel updated deliberately: 80 -> 104 bytes
  (MIN_VTABLE_SIZE pinned at the v4.0 baseline of 80).

SDK C++ helpers:
- DataSourceRuntimeHostView::pushMessage template: wraps a C++ closure
  (returning either 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.

Canonical-object blob serialization:
- Flat byte layout for Image / CompressedImage / PointCloud crossing
  the C ABI. Writer/reader pair under sdk/detail/.

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 and 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.

Status: design sketch posted as a draft. Compiles cleanly with the
companion parser/runtime work; not yet exercised end-to-end against
real data sources.

…functional scalars

The canonical-object pipeline and the pure-functional scalar path are C++
SDK contracts: MessageParserPluginBase::parseObject() and parseScalars()
are called directly on the C++ pointer by the in-process runtime host,
preserving zero-copy via BufferAnchor. The C ABI slots (parse_object,
parse_scalars) and their wire-format support are removed; pure-C plugins
emit scalars via the parse() slot writing to writeHost.

Removed:
- detail/canonical_object_serialization.hpp (full file).
- PJ_canonical_object_blob_t and PJ_named_field_value_buffer_t from
  canonical_object_abi.h.
- parse_object and parse_scalars slots from PJ_message_parser_vtable_t.
- trampoline_parse_object and trampoline_parse_scalars.
- Per-instance buffers (scalars_owned_buf_, scalars_abi_buf_,
  object_blob_buf_) that only the trampolines used.

Kept:
- SchemaHandler::parse_scalars / parse_object (C++ callables registered
  by plugins) and MessageParserPluginBase::parseScalars / parseObject
  (C++ methods invoked by the host).
- classify_schema C ABI slot (used by MessageParserHandle::classifySchema).
- The parse() slot for pure-C plugins that emit scalars to writeHost.

Vtable size: PJ_message_parser_vtable_t shrinks from 104 to 88 bytes
(80 v4.0 baseline + 1 tail slot for classify_schema).
PJ_MESSAGE_PARSER_MIN_VTABLE_SIZE stays pinned at 80.