[p8-s1] Lowering concrete scenarios to the IR

Pillar: P8 — OpenSCENARIO DSL Execution · Roadmap: docs/roadmap/roadmap.md § p8-s1

Goal

.osc in, Scenario IR out.

Deliverables

• Frontend (frontends/dsl/lowering/): scenario entry point selection (implementation-defined per DSL §7.7.2 — top-level scenario by qualified name, CLI/API parameter).
• Concrete parameter binding (equality/single-value keeps resolved at init per §7.3.11; anything else diagnosed).
• Actor → IR entity mapping (vehicle/person physical fields → p2-s1 taxonomy).
• §8.8 movement-action subset (move, drive, walk, assign/change/keep speed and position family, follow_lane, change_lane, gap/headway keeping, follow_path/follow_trajectory per matrix) → IR actions.
• set_map_file → road backend wiring; diagnostics for non-concrete remnants (§-citations); scena-run accepts .osc.

Tests

• dsl_lowering_test.cpp (IR snapshots for self-authored spec-derived concrete scenarios; non-concrete red fixtures).

Docs

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

Exit criteria

• IR snapshot suite green
• A trivial DSL scenario runs end-to-end
• Single focused PR merged with CI green (macOS/Linux/Windows, sanitizers, format)

Depends on: p7-s5, p5-s6