[p7-s1] DSL grammar & lexer (ANTLR4)

Pillar: P7 — OpenSCENARIO DSL Frontend · Roadmap: docs/roadmap/roadmap.md § p7-s1

Goal

Tokens and layout exactly per DSL §7.2.1.

Deliverables

• Frontend (frontends/dsl/): approved ANTLR4 dependency wiring.
• Lexer: identifiers incl. escaped |...| form, physical literals with units, int/uint/hex/float/bool/string literals, # comments, line continuation.
• Indentation preprocessor (offside rule → INDENT/DEDENT, CRLF-safe, per §7.2.1.4).
• std::from_chars numeric conversion.

Tests

• dsl_lexer_test.cpp (token goldens incl. pathological indentation and unit literals).

Docs

• Frontend README (grammar written from spec §7.2 — clean-room provenance note).

Exit criteria

• Lexer goldens green
• ANTLR4 dependency pinned + licensed
• Single focused PR merged with CI green (macOS/Linux/Windows, sanitizers, format)

Depends on: p1-s1, p1-s4; maintainer ANTLR4 dependency approval before start (open question OQ-2)