feat(qdrant): add Qdrant vector backend

[n0rmanc]

Description

Adds an optional Qdrant REST vector backend while preserving the existing CollectionAdapter / ICollection contracts and upper-layer search APIs. The implementation uses the Python standard library and does not add qdrant-client as a dependency.

Human Involvement

• A human participated in the implementation or review loop
• This PR was generated entirely by AI agents without human participation in the loop

Related Issue

None. This PR introduces the Qdrant backend described in the repository design/implementation plan.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• Added an injectable urllib-based Qdrant REST transport with API-key support, bounded errors, and configurable timeout.
• Added Qdrant collection lifecycle, metadata sidecar, schema/index persistence, dense CRUD/count/search, deterministic UUID5 point IDs, and path/filter mapping.
• Added durable sparse-term dictionary with positive uint32 indices, collision detection, sparse encoding/decoding validation, and dense-only guards.
• Registered the qdrant backend, added nested configuration, English/Chinese documentation, adapter documentation, and environment-gated live integration coverage.
• Kept Contains/content grep semantics explicit and unsupported in Qdrant; filesystem grep fallback remains available.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Focused verification:

• tests/storage/test_qdrant_adapter.py: 45 passed
• transport/lifecycle subset: 2 passed
• sparse subset: 9 passed
• disposable live Qdrant integration: 1 passed against local Qdrant 1.x container
• Ruff check and git diff --check: passed
• The broader tests/storage tests/unit collection remains red due known baseline failures (19 reproduce on local main, with additional environment/order-dependent failures); these are documented as a repository-health boundary rather than Qdrant-specific failures.

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published (no dependent changes are required)

Screenshots (if applicable)

Not applicable.

Additional Notes

• Qdrant remains an optional backend; no new mandatory dependency was added.
• The implementation branch is feat/qdrant-integration and is based on the repository main branch.
• The implementation plan has all 19 task checkboxes marked complete with verification notes.
• This PR is intentionally opened as a draft for review.