feat(index): add SeekDB derived-index backend

[knqiufan]

Summary

Adds SeekDB as a third rebuildable derived-index backend while preserving
LanceDB as the default and keeping Markdown as the source of truth.

• route all seven typed index repositories and lifecycle operations through a
  new SeekDB adapter;
• support embedded SeekDB on Linux/macOS and remote seekdb Server or OceanBase
  connections, with optional dependency extras and validated configuration;
• map portable schemas to typed scalar, JSON, whitespace-tokenized full-text,
  and cosine HNSW vector columns/indexes;
• provide safe SQL rendering, exact UTC epoch-millisecond conversion, batched
  writes, native pagination, BM25 search, vector search, and strict column/index
  drift diagnostics;
• guard embedded directory ownership, retain connection locks until cancelled
  driver calls finish, restore physical-session settings after reconnects, and
  suppress raw driver exception chains in application logs;
• integrate cascade rebuild, configuration examples, architecture/storage
  docs, changelog, unit/contract/integration/e2e fixtures, and both embedded and
  latest remote SeekDB CI jobs.

Closes #448.

Area

• Architecture method
• Benchmark
• Use case
• Documentation
• Developer experience
• CI, build, or release

Verification

• Local Windows validation: 81 passed for SeekDB unit tests,
  and configuration tests.
  A process-local fcntl import shim was used only to collect portable tests;
  actual flock calls still raise. Real-engine tests require Linux/macOS or a configured remote endpoint and
  were not executed locally.
• Added cancellation/query/shutdown ordering, repeated cancellation, failed ping,
  physical connection replacement, and caller-side exception-log privacy tests.
• Real-engine validation also covers SeekDB internal full-text/vector catalog
  metadata without treating engine-generated backing indexes as schema drift.
• Added a real remote regression that replaces the physical connection with
  NO_BACKSLASH_ESCAPES enabled and verifies text round trips after recovery.
• Passed repository-wide Ruff lint/format, all four import-linter contracts,
  asset/file-size gates, deprecated-name/contributor-doc gates, datetime discipline,
  and documentation checks.
• Real-engine checks passed on the latest commit in the
  fork CI run:
  • Embedded SeekDB 1.4: 25 passed, 1 remote-only test skipped for repository/
    port contracts; 27 passed for the tiered API suites.
  • Remote SeekDB latest: 26 passed for repository/port contracts, including
    the connection replacement regression; 27 passed for tiered API suites.
  • Lint, package build, and the complete Python 3.12/3.13 unit and general
    integration suites passed.
  • The existing Milvus job fails before tests because the upstream compose
    references an unavailable minio/minio image; this run is not fully green.
• The CI workflow supports manual dispatch for fork validation. Upstream PR
  workflows require maintainer approval; pending approval is not a passing check.

Checklist

• I kept the change scoped to the relevant area.
• I am opening this from a separate branch, not pushing directly to main.
• I updated docs, examples, or setup notes when behavior changed.
• I added or updated tests when the change affects behavior.
• I did not commit secrets, .env files, dependency folders, or generated output.
• Active relative links in Markdown files resolve.

Notes for Reviewers

The adapter isolates the SQL-oriented pyseekdb 1.4.x client surface in one
connection manager and pins the optional dependency below 1.5 so future client
surface changes cannot silently alter the contract. Schema drift is fail-fast
because every physical table is disposable and rebuilt from Markdown with
everos cascade rebuild.

The default install and default [index] backend = "lancedb" behavior are
unchanged.

By submitting this pull request, I agree that my contribution is licensed under
the Apache License 2.0.