refactor(server): address PR #16 review feedback

Hoshe + Tyre review items:
- Use StableId consistently for wire entity_id (H4) across observer,
  observation, interpretation, and interaction systems
- Make NearbyInteractionBuffer.interactions private with take() (H1/H20)
- Add system ordering constraint for compute_nearby_interactions (H5)
- Panic on missing PlayerCharacter in input processing (H2)
- Remove redundant paused field from GameTime (Tyre8)
- Remove #[serde(default)] from nearby_interactions (H3)
- Change NearbyInteraction.distance from f32 to u32 (H8)
- Add sort stability for equal verb priorities (H6)
- Scope constants to pub(crate) (H7)
- Add debug_assert for last_observed_tick ordering (H10)
- Strengthen unregistered entity handling to debug_assert + error (H11)
- Document fractional tick accumulation (Tyre9)
- Extract collect_remembered_entities helper (Tyre2/H17)
- Add half_rate_no_drift_over_10000_frames test (H14)
- Add mid-range and deterministic sort tests (H15)
- Add fixture version assertion (H16)
- Regenerate msgpack fixtures for wire format changes

146 unit + 19 integration tests pass, zero clippy warnings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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