grammar: SQL expression grammar fragment (ADR-0031)

A new `src/dsl/grammar/sql_expr.rs` authored as a parallel
fragment to `expr.rs` (the DSL `WHERE` grammar, ADR-0026). The
ADR's stratified ladder lands as named `static` `Node`s, one per
precedence tier:

  or_expr → and_expr → not_expr → predicate → additive →
  multiplicative → unary → primary

Recursion through `Node::Subgrammar` reuses ADR-0026's
`MAX_SUBGRAMMAR_DEPTH = 64` cap unchanged; no new walker
capability is required. `predicate_tail` follows ADR-0026's
factoring (shared operand prefix, infix `NOT` as an explicit
branch, no `Optional`-first branch) so `Choice` discriminates
cleanly. `name_or_call` factors the identifier-prefix shared
between column refs and function calls into a single `Ident`
followed by an `Optional` `( call_args )` tail — the same
hazard-avoidance shape `predicate_tail` uses.

The fragment exports `pub static SQL_OR_EXPR` (test entry) and
`pub static SQL_EXPRESSION` (drop-in `Subgrammar(&SQL_OR_EXPR)`
that SQL `CommandNode` shapes embed in their `Seq`). No AST
builder — every Phase-1 consumer (SELECT projection, WHERE)
runs validated SQL as text per ADR-0030 §4/§6.

13 unit tests cover every operator and precedence pair, the
full predicate set, `CASE` (searched + simple) including
`count(*)` and `count(distinct …)`, parenthesised regrouping,
case-insensitive keywords, the depth cap, and a representative
set of malformed inputs that do *not* walk.

Module registered via one new line in `grammar/mod.rs`.

src/dsl/grammar/mod.rs

@@ -27,6 +27,7 @@ pub mod data;
 pub mod ddl;
 pub mod expr;
 pub mod shared;
+pub mod sql_expr;
 
 use crate::dsl::command::Command;
 use crate::dsl::walker::context::WalkContext;