XY-792: [ELF hardening 4/4] Add Docker Compose local dev stack

[yvette-carlisle]

Summary

• Add a local Docker Compose dependency stack for Postgres with pgvector and Qdrant.
• Add a strict-valid local Docker config and tokenizer fixture for deterministic local smoke paths.
• Update startup docs and README to route local bootstrap through the Compose stack.

Verification

• docker compose -f docker-compose.yml config
• cargo test -p elf-config docker_local_config_is_strict_valid
• cargo test -p elf-chunking loads_local_dev_tokenizer_fixture
• cargo make fmt
• cargo make lint-fix
• cargo make checks
• semantic drift audit: pass