refactor: split StructuralAnalysisSystem into connectivity + stress systems

[ThreeIce]

What to build

Split \JerikoStructuralAnalysisSystem\ into two independent systems to align with the logical module boundary between connectivity/state determination and stress computation.

System A: Connectivity Analysis (belongs to Connection Graph module)

• Phase 1: Map dirty elements to dirty islands
• Phase 2: Multi-source BFS from boundary Anchors, assign Layers, determine reachable/unreachable
• Phase 3: Dynamic cleanup (transition unreachable elements)
• State transitions (Static/Dynamic) and state change events

System B: Stress Computation (belongs to Structural Stress module)

• Only runs when topology changes occur — not every frame
• Scoped to affected islands only (minimize computation surface)
• Collect dirty island members, read Layer from JerikoElement component
• Top-down weight accumulation (highest layer to lowest)
• Stress computation: max(0, accumulatedLoad - ownMass) * coefficient
• Write AccumulatedLoad and Stress to JerikoStructuralStress component
• Specific triggering/scheduling mechanism TBD at implementation time

Dependency: A triggers B

System A and System B are decoupled. When System A completes topology rebuild for dirty islands, it signals (via event or dirty flag) that stress recomputation is needed for those islands. System B reacts to this signal and only recomputes stress for the affected islands. If no topology changes occur in a given frame, System B does not run. Cached stress values persist between frames for unaffected islands.

Rationale

Currently the single IslandAnalysisJob mixes graph-topology operations (BFS, reachability) with mechanical analysis (weight flow, stress). Splitting them:

• Aligns code structure with logical module boundaries
• Makes each system independently testable
• Enables stress computation to be skipped entirely on frames with no topology changes
• Minimal performance impact: BFS (the expensive part) runs once in System A; System B only does linear iteration + arithmetic on dirty islands

Acceptance criteria

• New system (e.g. JerikoStressComputationSystem) handles weight accumulation and stress calculation
• JerikoStructuralAnalysisSystem no longer contains weight/stress logic
• System B only executes when System A signals topology changes (not every frame)
• System B is scoped to affected islands only — unaffected islands retain cached values
• New system reads Layer from JerikoElement component (written by System A)
• All existing structural stress integration tests pass without modification
• No measurable performance regression (profile with 10-building stress test scene)

Blocked by

None - can start immediately.