grammar: WHERE-expression fragment + Expr AST + build_expr (ADR-0026 step 2)

The stratified WHERE-expression grammar — or / and / not /
bool_primary / predicate tiers as named `static` Node
fragments, recursing through `Subgrammar`. Covers the six
comparison operators (`<>` and `!=` both NotEq), AND / OR /
NOT, parentheses, LIKE / IN / BETWEEN with optional infix NOT,
and IS [NOT] NULL. `predicate_tail` factors the shared operand
prefix and the infix NOT so the Choice branches discriminate
on a cleanly-failing first token.

New recursive Expr / Predicate / Operand / CompareOp AST in
dsl::command. `build_expr` folds the flat matched-terminal
slice into an Expr — a deterministic recursive descent
mirroring the grammar tiers, with single-child tiers
collapsing. Per ADR-0026 §3 option 1: the walker stays a pure
structural matcher; Expr is assembled only in this
submit-time fold.

Fragment + builder are unit-tested standalone (walk against
&OR_EXPR, then build_expr); not yet wired into any command.

src/dsl/grammar/mod.rs

@@ -25,6 +25,7 @@
 pub mod app;
 pub mod data;
 pub mod ddl;
+pub mod expr;
 pub mod shared;
 
 use crate::dsl::command::Command;
@@ -312,7 +313,6 @@ pub enum Node {
     /// builds a fresh node from the `WalkContext` at walk time;
     /// this one references a fixed fragment already in the
     /// grammar tree.
-    #[allow(dead_code)]
     Subgrammar(&'static Self),
     /// Resolves at walk time using the active `WalkContext`.
     /// Phase D+ uses this for `column_value_list`. The factory