core: introduce TeamSide enum to replace team-name string keying

[valmathieu]

Problem

Team identity is currently the free-form string "North-South" / "East-West", and it leaks across layers the same way position strings do:

• All score bookkeeping is keyed by team.name: Game.scores (packages/contrai-engine/src/contrai_engine/model/game.py), Round.team_tricks / round_scores (model/round/round.py), and the whole of model/round/scoring.py passes team_name strings around.
• Game.winner is a team name string, not a team.
• The view re-derives team from position strings (view/formatting.py — position in ("North", "South")) and keys TEAM_ABBR on name strings (view/theme.py).
• Team (packages/contrai-core/src/contrai_core/team.py) has no __eq__/__hash__, so call sites silently rely on identity semantics.

Proposal

• Add a two-member TeamSide enum (NS, EW) in contrai-core, plus a Position.team_side property (North/South → NS, East/West → EW). Depends on the planned Position enum.
• Rekey score dicts as dict[TeamSide, int]; team display strings ("North-South", "N-S") become presentation mappings in the view.
• Cleanup rider: Team.get_partner and Team.contains_player are dead code in production and become fully redundant once Position.partner exists — remove them.

Why (roadmap)

Training rewards (RL), scraped-game labels (supervised learning), and the future web server's wire format all need a stable, serializable team identity. Name strings that happen to match dict keys break silently once games are persisted.

Acceptance criteria

• No "North-South"/"East-West" literals outside view presentation mappings and prose.
• Scoring, Game.scores, Round.team_tricks keyed by TeamSide with tests updated.
• pytest green in both core and engine packages.