refactor(ranker): split ranking helpers into focused modules

[RaghavRD]

Move ranking orchestration, filters, quality scoring, source trust constants, and candidate variant selection into focused modules.

Keep whichllm.engine.ranker as a compatibility shim so existing public and test imports continue to work.

Refs #41

What

• Move rank_models orchestration into whichllm.engine.ranking
• Move model/profile/evidence filters into ranking_filters
• Move quality scoring and family selection logic into ranking_score
• Move source trust constants into ranking_sources
• Move candidate GGUF/synthetic variant logic into ranking_variants
• Preserve old imports through whichllm.engine.ranker

Why

This continues the #41 core-module refactor by splitting the large ranker module into focused pieces without changing the public import surface.

This is the penultimate major #41 refactor slice. After this, the remaining large core module is src/whichllm/cli.py.

Testing

• Focused ranker/regression tests pass (74 passed)
• CLI monkeypatch compatibility tests pass (6 passed)
• Ruff check passes
• Ruff format check passes
• Full suite except environment-sensitive run smoke test passes (465 passed, 1 deselected)

Notes

Full pytest in my sandbox hit test_run_exits_gracefully because model fetching returned a DNS/network error message from the environment. The ranker-specific and compatibility coverage passes.

[Andyyyy64]

really sorry this took me so long to review, and thanks for your patience here.

I retested this on the current main, including the changes that landed while the PR was waiting. the split preserves the ranking logic, the compatibility shim keeps the existing imports working, and the new module boundaries look much easier to work with.

local verification on current main:

• Python 3.11: 484 passed
• Python 3.12: 484 passed
• Python 3.13: 484 passed
• Ruff lint and format checks passed

looks good to me — thanks for taking on this refactor.