partial writes + CachedOntology + route validation

Addresses the five concrete gaps the brutally-honest review on #309
called out:

  1. arrow_type_for_semantic no longer collapses everything to Utf8.
     Currency → Float32, Date(_) → Date32, CustomerId / InvoiceNumber
     → UInt64. DataFusion can now do real numeric / temporal predicate
     pushdown on those columns. The remaining semantic types stay
     Utf8 by deliberate choice — round 3 may pivot specific ones (Geo
     → struct{lat,lon}) when a consumer asks.

  2. CachedOntology helper extracted upstream. Bundles an Arc<Ontology>
     with eagerly-projected DTOs per Locale (De/En). Prevents the
     per-call OntologyDto::from_ontology rebuild that medcare-rs's
     MedcareOntology and smb-office-rs's session ontology both grew
     independently. One implementation, one bug surface.

  3. validate_route(route, ontology) added to parallelbetrieb. Parses
     /api/{entity_type}/{...} and asserts entity_type resolves to a
     declared Schema.name (case-insensitive). 4 tests covering
     accept-valid, reject-typo, reject-missing-prefix, reject-empty.
     Used as a pre-flight check for static route lists; the runner
     itself doesn't gate on it because typo routes are still genuine
     drift telemetry.

  4. from_columns_partial added for PATCH-style upserts. Allows
     omitted Optional / Free columns (filled with Arrow null arrays);
     still rejects missing Required columns and undeclared columns.
     The existing from_columns keeps its strict full-row contract.

  5. route_for_column now reads OuterColumn.codec_route directly,
     copied from the upstream PropertySpec.codec_route at schema
     derivation. Round-1 went through route_tensor which is
     calibrated for model-weight tensor names (q_proj, lm_head, ...)
     and would silently mis-classify document predicates. The
     contract's own field is now the source of truth — drift-by-
     construction impossible.

Adds 7 new tests (6 in transcode/ + 1 implicit in cam_pq_decode
where the existing test was rewritten to assert the new semantics):
  - cached_ontology_projects_every_locale_at_construction
  - cached_ontology_clones_are_arc_cheap
  - cached_ontology_inner_round_trips
  - validate_route_accepts_known_entity_type
  - validate_route_rejects_typo_entity_type
  - validate_route_rejects_missing_api_prefix
  - validate_route_rejects_empty_entity_segment
  - route_for_required_scalar_column_uses_property_spec_default
    (rewritten from route_for_scalar_columns_skips_codec; old test
    asserted the wrong thing — required scalars should default to
    Passthrough, not Skip)

Verified:
  cargo check across {default, query-lite, query-lite+audit-log+
    auth-rls-lite} — clean
  cargo test transcode:: → 33/33 passed (was 26/26 in #309; +7
    new + 1 rewritten)
  cargo clippy on the same combos — zero transcode warnings
  rustfmt --check — clean across all 4 modified files