SP2b.1: Dispatcher interface + DispatchDecision (core/commonMain)

[bedaHovorka]

Sub-issue of #538 (SP2b Dispatcher agent)

Objective

Define the pluggable Dispatcher contract (requestRoute(train, destination): DispatchDecision) and DispatchDecision (selected path, switch commands, signal commands, human-readable rationale) in core/commonMain. (#532 Technical Design §1.)

Depends on

SP0, SP1.

Design context: see the GOAL_10 Agent-Based Traffic Control comment on #532 — crucial reading.

[bedaHovorka]

Scope narrowed — partial pull-forward by #729 (SP0.7)

Per the approved SP0.5 design spec (docs/specs/2026-07-08-544-sp05-drive-loop-design.md §3.1), #729 (SP0.7 — "Reshape Dispatcher to a pure decide() seam + DispatchObservation") partially pulls this issue forward: it defines the base pure Dispatcher.decide(observed): List<DispatchDecision> seam and the minimal DispatchDecision subtypes the lifted AgentLoopDriver needs now, plus DispatchObservation.

This issue (#556) keeps ownership of the LLM-specific decision richness layered on top later in the SP2b timeframe — it is not superseded, just narrowed. #557 (SP2b.2) similarly remains the owner of the PathFinder/ConflictResolver-driven rule richness; #729 only reshapes the existing minimal RuleBasedDispatcher's call signature.

See #729 for the base-seam implementation and #535 for the full SP0.7–SP0.12 breakdown of the SP0.5 implementation.