Port vocabulary module to dev (and add tests)

[zeemscript]

Scope

Files to port from main: vocabulary.py.
Tests: none exist on main — write unit tests covering the module's public functions.
Wiring: register the router in main.py.

Background

During the last campaign, dnb-ai's main and dev branches diverged and bounty PRs landed on both. We are consolidating on dev as the single source of truth. This issue ports one self-contained module that currently exists only on main over to dev, together with its tests, so dev becomes the complete app.

Rules (must follow for the PR to be payable)

• Branch from dev; your PR must target dev (not main).
• Copy the reference implementation from main — do not rewrite it.
• Bring the module file(s) and its test(s). pytest for the module must pass and flake8 must be clean.
• If the module exposes a FastAPI router, register it in dev's main.py exactly as main does — this is the only change permitted to a shared/core file, and it must be minimal.
• Do NOT modify any reconciliation-reserved core file: orchestration.py, main.py (beyond the single router include), tafsir.py, citations.py, cross_reference.py, confidence.py, review.py, verifier.py, model_router.py, query_optimizer.py, history.py, faraid.py, thematic_quran.py, prompts/registry.py.

Acceptance criteria

• Module file(s) ported from main to dev unchanged in behavior
• Test(s) ported (or written, where noted) and passing under pytest
• flake8 clean
• Router (if any) registered in main.py and endpoint reachable
• PR targets dev, no changes to reserved core files