Collapse BRA/ARG/USA producers into one PDL-derived producer archetype

[Hackatro]

Raised in #31 (topology.py ID_OVERRIDES / FARMER* fragments).

After #31 the three producer archetypes (bra_farmers, arg_farmers, usa_farmers) are behaviorally identical — all ROLE_PRODUCER, one unified _step_producer. They survive as three hand-maintained entries (two ID_OVERRIDES + one KIND_ARCHETYPES default) purely for data/wiring identity, not behavior. Adding a new producing region still needs a hand-written override + param fragment + per-region scenario knobs.

Fix: collapse them into a single generic producer archetype differentiated only by PDL entity id, so a new (region, agriculture) entity dropped into the PDL wires itself with no topology.py edit. (build_roster already has the per-entity split machinery — its outputs just don't match today's schema yet.)

Three blockers, each must become PDL/scenario-derived:

1. Producer list names ARE the output schema — bra_farmers → CSV/DB tables + FLOW_ADJACENCY keys. Collapsing → entity-id-based names → schema change. (Generalize port routing with origin allow-list #19 frees the wholesaler volume columns but not the producer table names.)

2. Fertilizer is a universal producer input, not a BRA special-case. Today _FARMER_BRA alone binds fertilizer → ("fertilizer_supply","price"); ARG/USA don't. Decision (per Unify Farmer & Trader into region-agnostic capability supersets (roles without regions) #31 follow-up): every producer depends on fertilizer, so the binding moves into the shared producer archetype and applies to all producers by default. This removes the need to derive it per-entity — it's a shared default, not a component to special-case. (Behavior change: ARG/USA fixed costs will now respond to fertilizer_supply price shocks — e.g. the ammonia_halt → fertilizer_supply events in the energy/compound cascades — so those scenarios need a re-baseline, same pattern as USA's unification in Unify Farmer & Trader into region-agnostic capability supersets (roles without regions) #31. brazil_drought is unaffected: it doesn't shock fertilizer.)

3. Per-region cost knobs are separate scenario fields (fixed_costs_bra_farmer, …). Needs an entity-id convention (fixed_costs_<entity_id>) + resolver.

Acceptance:

• No per-producer entries in ID_OVERRIDES (brazil_farms/us_farms removed); one generic producer archetype

• A new (region, agriculture) PDL entity becomes its own producer list with no topology.py edit

• Fertilizer binding lives on the shared producer archetype (all producers bind it), not on any single region

• Producer params sourced by entity-id convention, not per-region scenario fields

• Existing scenarios reproduce current producer values byte-for-byte (pure refactor — behavior unchanged; only table names may move)

Depends on: #19 (schema thaw for the volume columns)
Ref: topology.py ID_OVERRIDES, _FARMER_BRA/_FARMER_ARG/_FARMER_USA, build_roster producer-split.