Declare geography and sea-crossing classifiaction in data

[Hackatro]

Problem

topology.py decides sea crossings from a hardcoded dict LOCATION_CONTINENT (6 countries). derive_sea_edges calls a stage "sea" when its endpoints sit on different continents.

Four flaws:

1. Silent skip. Guard if c_src and c_dst: — an unknown location yields no
   crossing, no error, no lane. A PDL with location: China loses its sea leg silently.
2. Joins on free text, not entity id. location: Brasilien instead of Brazil
   misses — silently, per (1).
3. Sidecar entities are invisible to the classifier (they carry no location:).
4. Continent-difference is a leaky proxy. UK→Netherlands (sea, same continent)
   reads as land; Panama→Colombia (land, two continents) reads as sea.

Geography is also split across three places: this dict, inline lat/lng on sidecar
entities, and the frontend gazetteer.

Solution

A top-level geo: block in the roster sidecar, keyed by entity id — the single home
for geography:

geo:
  brazil_farms:   { continent: South America, lat: -15.8, lng: -47.9, illustrative: true }
  rotterdam_port: { continent: Europe,        lat:  51.9, lng:   4.1, illustrative: true }

Plus a per-stage override for the cases the proxy gets wrong — a dedicated top-level
crossings: block, normalized (undirected) pair key, binary mode:

crossings:
  - stage: [uk_port, rotterdam_port]   # [a,b] == [b,a]
    mode: sea
  - stage: [panama_hub, colombia_hub]
    mode: land

Resolution order: explicit stage mode → both endpoints' geo.continent → raise.

Scope

• Add geo: to load_roster_sidecar, validated like _entity_placement
  (type / range / finiteness / boolean flag).
• Make geo: the single home for geography: move inline lat/lng/illustrative
  off the sidecar entities into it, delete the inline path; _entity_placement
  reads from geo:.
• Rewrite derive_sea_edges to resolve continents from geo: over the merged
  entity list, by id.
• Add the crossings: override (shape decided above).
• Fail loudly — an unresolvable crossing raises, naming entity and stage.
  Never a silent skip.
• Delete LOCATION_CONTINENT.
• Verify whether replacement_crossings in build_flow_adjacency can be simplified
  once classification reads the merged list. Don't remove on assumption.

Acceptance

• No geographic literal in topology.py; geography declared in exactly one place.
• An unresolvable crossing raises (entity + stage named); never yields a missing lane.
• Shipped and alternate PDLs: unchanged topology, unchanged values.

Notes

• Sidecar is an additive bridge; fields must survive going upstream to the partner
  schema. continent: South America and mode: sea can; is_sea_side: true cannot.

Rejected

• Overloading sidecar edges: with a mode field — a PDL stage isn't an edge.
• A node flag on geo: (is_sea_side:) — repeats the node→edge inference mistake.
• Deriving continent from coordinates via a geo library — the coords are illustrative.

Out of scope

Frontend gazetteer migration (settled in #33) · #37 cardinality work.