Generalize port routing with origin allow-list

[Hackatro]

Generalize origin routing and remove region from the agent layer

Problem

Region (bra/arg/usa) is hardcoded across three places that should be
region-agnostic:

• Transport (transport.py, _move_split): Brazilian ports filter out
  non-BRA soy with hardcoded exclude_arg / exclude_usa flags. Logic is
  negative ("all except…") and leans on untyped getattr reads. Every new
  origin needs another exclude_* flag. (Raised in PR
  4-allow-for-conditional-events.)
• Wholesaler agent (trader.py): derives bra/arg/usa_volume from
  volume_by_origin using hardcoded origin strings — location living in an agent.
• Reporting (data_collector.py): the per-region CSV columns require those
  named attrs to exist on the agent, because Melodie's DataCollector records by
  attribute name.

volume_by_origin (keyed on farmer.origin) is already the region-agnostic
source of truth. Region is a routing + reporting dimension, not agent state.

Fix

• Give each export port an explicit accepted_origins set (Santos/Paranagua →
  {"bra"}) and route only matching-origin volume from volume_by_origin.
  Drop the exclude_* params and getattr lookups.
• Remove bra/arg/usa_volume and all origin string literals from trader.py.
  The agent keeps only volume_by_origin.
• Move the origin→region-column projection into the reporting layer, computed
  from volume_by_origin. This is the only place region may live.
• Keep the origin→region mapping in exactly one non-agent location.

Acceptance

• No exclude_* flags in transport.py; routing driven by accepted_origins.
• No region attrs or origin/region string literals in agents/*.py
  (grep clean: bra_volume|arg_volume|usa_volume,
  brazil_farms|argentina_farms|us_farms).
• Transport reads volume_by_origin, never named attrs.
• Per-region CSV columns produced by the reporting layer; names/order unchanged.
• Origin→region mapping lives in exactly one non-agent location.
• Adding a new producer origin needs no change to _move_split and no change
  to any agent file.
• Full-run regression: all Result_Simulator_*.csv byte-identical to a
  pre-change baseline (Santos/Paranagua output unchanged).

Note

Branch 31-unify-farmer-trader-… merges first with the compatibility shim in
place (trader.py:140-144, plus the bra/arg/usa_volume attrs). This ticket
then pulls dev and removes that shim — deleting the three attrs, the origin
string literals, and the now-stale comment — as part of the work here.

Ref: transport.py:_move_split, trader.py, data_collector.py