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 and recover remote sessions after
  disconnects without logging SQL or credentials;
• 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

83 passed
  pytest: SeekDB unit tests, derived-index contracts, configuration

144 passed
  pytest: SeekDB + Milvus regression, routing, configuration, cascade CLI

81 passed, 4 skipped; 81.07% branch coverage
  SeekDB package coverage (real-engine cases skip locally without an endpoint)

Passed
  ruff check + format check
  all four import-linter architecture contracts
  repository asset and file-size gates
  deprecated-name and contributor-doc gates
  datetime-discipline and OpenAPI drift gates
  documentation links and issue-template YAML
  uv lock --check
  sdist/wheel build, top-level import smoke test, and wheel-content check
  pyseekdb 1.4.0.post1 runtime API/signature inspection

The PR adds Linux CI coverage for the locked embedded 1.4 runtime and a
separate oceanbase/seekdb:latest service job for the remote adapter. Both run
the repository/port contracts and tiered API suites.

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.

[knqiufan]

CI, Docs, and Commit lint are currently awaiting the repository's required maintainer approval for workflows from a fork. Once approved, I will monitor the runs and address any failures on this branch.