Engine: schema-forge-cel crate — CEL lexer + parser → AST

[rrrodzilla]

Workstream: Engine · Part of #89

Context

Per the #91 decision, we build our own minimal CEL evaluator over DynamicValue — no upstream/vendored crate, no Cedar. First piece: the parser.

Scope

• New crate crates/schema-forge-cel.
• Lexer + parser for the CEL expression grammar (cel-spec langdef / Cel.g4) → a typed Rust AST.
• Cover the surface the rules engine needs: literals (incl. uint/bytes/duration/timestamp forms), identifiers, field selection, indexing, function & method calls, unary/binary/ternary operators, list/map construction, and macro call syntax (has, all, exists, exists_one, map, filter).
• Parse errors with position info.

Acceptance

• Passes the parse section of the Spike: measure pure-Rust CEL conformance against the official cel-spec suite #90 conformance oracle.
• AST unparses for debugging.

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Dependencies: Built against the #90 oracle (parse section). Blocks the evaluator core. Soft.

[rrrodzilla]

Landed in a675303 — feat(cel): add CEL lexer + parser → typed AST (#107). The schema-forge-cel crate ships the lexer and a recursive-descent parser producing the typed ast::Expr, with unparse round-tripping. Closing; code merged on dev.