[p8-s2] Composition operators: serial, parallel, one_of

Pillar: P8 — OpenSCENARIO DSL Execution · Roadmap: docs/roadmap/roadmap.md § p8-s2

Goal

DSL §7.6.2 invocation semantics on the runtime's storyboard machinery.

Deliverables

• Kernel: composition nodes mapping onto the P1 execution model (designed for in p1-s1's lowerability review) — serial segmentation (member starts when predecessor ends, §7.6.2.1.2); parallel with default start overlap and all-members-complete join (§7.6.2.1.4); one_of deterministic selection input (engine API + --select, default first alternative, §7.6.2.1.3).
• duration bounds; until (terminates the annotated invocation exactly at the event, §7.6.2.5.4).
• wait/emit/on plumbed onto the trigger system (§7.6.2.5; emit is zero-time, wait is pure synchronization).
• Predefined start/end/fail events (§7.3.10.3) mapped to storyboard element transitions.

Tests

• dsl_composition_test.cpp (phase-boundary traces per operator, nested compositions, event-gated phases).
• Determinism fixture per alternative.

Docs

• Coverage matrix rows updated with spec citations (docs/roadmap/coverage/).

Exit criteria

• Operator suite green
• GS-13's body runs via the API
• Single focused PR merged with CI green (macOS/Linux/Windows, sanitizers, format)

Depends on: p8-s1