[p8-s3] Movement modifiers

Pillar: P8 — OpenSCENARIO DSL Execution · Roadmap: docs/roadmap/roadmap.md § p8-s3

Goal

The declared DSL §8.9 modifier set with concrete values.

Deliverables

• Frontend + kernel: modifier application per §7.3.12.4 (argument = equality constraint) compiled onto the P2/P5 motion machinery.
• In-scope set per matrix: position/keep_position; speed/change_speed/keep_speed; acceleration; lane/change_lane (side required for determinism — unspecified side is diagnosed, not randomized)/keep_lane; lateral; along/along_trajectory; distance.
• at phase anchoring (start/end/all, §8.9.19).
• No parallel modifier machinery in the runtime — modifiers lower to existing IR actions/constraints wherever expressible, with any kernel addition documented and spec-cited.

Tests

• dsl_modifier_test.cpp (each modifier → expected IR/trace, combination fixtures, conflict diagnostics).

Docs

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

Exit criteria

• Modifier matrix green
• Coverage matrix rows flipped
• Single focused PR merged with CI green (macOS/Linux/Windows, sanitizers, format)

Depends on: p8-s2