Add REST API server, Rust client SDK, and point lookup

[beinan]

Summary

• Introduces three new crates to enable lance-context as a remote service:
  • lance-context-api: Shared request/response DTOs (serde-only, no heavy deps)
  • lance-context-server: Multi-context axum HTTP server with 13 REST endpoints under /api/v1/
  • lance-context-client: Typed Rust HTTP client SDK (no arrow/lance transitive deps)
• Adds point lookup (get by ID) across all layers: core Rust, PyO3 bindings, Python sync/async API, and REST endpoint

REST API Endpoints

Method	Path	Purpose
POST	/contexts	Create a named context
GET	/contexts	List all contexts
GET	/contexts/{name}	Get context info
DELETE	/contexts/{name}	Delete context
POST	/contexts/{name}/records	Add records (batch)
GET	/contexts/{name}/records	List records (paginated)
GET	/contexts/{name}/records/{id}	Get record by ID
POST	/contexts/{name}/search	Vector similarity search
GET	/contexts/{name}/version	Current version
POST	/contexts/{name}/checkout	Time-travel to version
POST	/contexts/{name}/compact	Trigger compaction
GET	/contexts/{name}/compact/stats	Compaction stats
GET	/health	Health check

Crate Architecture

lance-context-api (shared DTOs: serde, chrono, base64)
       /                          \
lance-context-server          lance-context-client
(axum, lance-context-core)    (reqwest only — no arrow/lance)

Test plan

• All 19 existing lance-context-core tests pass
• cargo build succeeds for all three new crates with zero warnings
• cargo check succeeds for PyO3 bindings with new get() method
• Manual testing of server endpoints with curl
• Integration test: client SDK against running server

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