Porting guide: derived variables belong in the datamodule, base bindings in the report

[florian6973]

Summary

Follow-up to #19 (the featurization backend), addressing two gaps a real port would hit. Docs only — template/docs/PORTING-A-MODEL.md.jinja, all changes inside the data_backend == 'custom_featurization' branch, so the mtd render is untouched.

Derived variables belong in the datamodule. The Step 3 mapping table read as if every source variable needed a predicate, which pushes a port toward inventing predicates for derived quantities (SF ratio, severity scores) — those match nothing, so the column is all-zero and the model is silently blind to a variable the report claims it has. Predicates are deliberately base quantities; deriving from them in datamodule.py is the intended flexibility, and porting a derived variable that way is a ported ledger row, not adapted or omitted. The only real finding is a base variable the dataset does not record.

Required predicates get written down, with evidence. Nothing previously required a port to state which concepts the model needs or how they were bound:

• Step 1 enumerates the input variable list (base vs derived) before any code, then prescribes the binding workflow: search every MEDS-DEV dataset's predicates.yaml (not just the target — another dataset still tells you the ecosystem's name for a concept), reuse upstream names, and flag each gap as locally written / contributed upstream (preferred: it is per-dataset knowledge) / not recorded by the dataset.
• Step 5 gains a required-concepts table (concept, source variable, kind, binding, origin, matched events) whose counts are copied from the patients artifact's featurization.match_counts rather than self-reported.
• Step 7 gains the matching checks, including that no base concept has zero matched events.

Also drops a duplicated model architecture row that #19 introduced into the Step 3 table.

Test plan

• uv run pytest tests/ -q — 85 passed
• Both backends render clean: the mtd render contains zero featurization mentions, no stray blank lines from the new conditional blocks, EOF intact
• ruff check / ruff format --check clean

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