feat: add lancedb 0.27.2 + pin lance =4.0.0 for exact version compat

lancedb uses exact `=` pins on lance (not semver ^). To add lancedb 0.27.2
as a dependency, all lance crates must be pinned to exactly 4.0.0 (not 4.0.1).

Changes:
  - lance/lance-linalg/lance-namespace/lance-arrow/lance-index: "4" → "=4.0.0"
    across lance-graph, callcenter, catalog, benches, holograph
  - NEW: lancedb = { version = "=0.27.2", optional = true } in lance-graph
  - NEW: feature `lancedb-sdk` gates the lancedb dep
  - Cargo.lock regenerated

lancedb is NOT in default features — opt-in via `--features lancedb-sdk`.
This gives the workspace access to the LanceDB table/connection API surface
that third-party tooling (DuckDB lance ext, Polars reader, Python/JS SDK)
expects, without forcing the dep on minimal builds.

Version compat matrix:
  lancedb 0.27.2 → lance =4.0.0, datafusion ^52.1, arrow ^57.2
  Our pins:        lance =4.0.0, datafusion 52,     arrow 57    ✅

Workspace compiles clean (4 pre-existing warnings, 0 errors).

https://claude.ai/code/session_01SbYsmmbPf9YQuYbHZN52Zh

Author: claude