Goal 3 SP3: Block-entry violation warnings

[bedaHovorka]

Goal 3 SP3: emit CollisionWarning.BlockEntryViolation when a train enters an already occupied block.

Parent: #610

Deliverables

Modified files:

• core/src/commonMain/kotlin/cz/vutbr/fit/interlockSim/objects/tracks/DynamicTrackBlock.kt — emit CollisionWarning.BlockEntryViolation before throwing on double-occupancy attempt
• core/src/commonMain/kotlin/cz/vutbr/fit/interlockSim/sim/collision/DefaultCollisionDetectionService.kt — listen for BlockEntryViolation and trigger auto-halt/train pause

Design note

DynamicTrackBlock.enter() keeps its existing requireSimulation(occupant == null) safety invariant — it still throws if the block is already occupied. Goal 3 adds an emit-before-throw: immediately before the collision check, enter() calls emitCustom(CollisionWarning.BlockEntryViolation(...)). The existing safety behavior (throw) is preserved, so existing tests keep passing.

Acceptance criteria

• A second train entering an occupied block emits a BlockEntryViolation warning before the existing requireSimulation failure.
• If autoHaltTrainOnViolation is true, the service calls the entering train's halt callback.
• Existing SimpleTrackEnterLeaveTest and DynamicTrackBlockTest still pass unchanged.

Tests

• BlockEntryViolationWarningTest

Quality gate

./gradlew :core:test :core:integrationTest :core:detekt :core:ktlintCheck

Depends on

#611 Goal 3 SP1

Downstream link

Goal 9 SP3 (#585) uses BlockEntryViolation as the worst-case signal for resolution ranking.