WHERE expressions: matrix cells + predicate_tail grammar fix (ADR-0026 step 6)

Adds tests/typing_surface/where_expression.rs — 9 matrix
cells for the complex WHERE / show-data limit typing surface:
operator candidates after an operand, AND / OR after a
predicate, NOT, BETWEEN / IN bounds, and `show data`
where / limit.

Writing the cells surfaced a grammar bug. `predicate_tail`'s
`[NOT] negatable` branch started with `Optional(not)`, and an
Optional-first `Seq` always "commits" — so on an incomplete
input the walker's `Choice` returned that branch's
`Incomplete` early and discarded every sibling branch's
expected set, dropping `is` and the comparison operators from
completion after a column. Fixed by splitting it into
explicit `NOT negatable` and bare `negatable` branches — no
`predicate_tail` branch starts with an `Optional` now. The
matched terminal sequence is unchanged, so `build_expr` is
untouched.

Docs: ADR-0026 gains an "As-built notes" section recording
the option-1 builder realization, its two deviations from the
§3 sketch, and the deferral of §7 diagnostic flagging to
ADR-0027. requirements.md C5a -> [x] (steps 1-4) with the
test baseline refreshed to 1079; CLAUDE.md's deferred list
reconciled (C5a implemented; the QA1/QA2 note now points at
ADR-0028).

tests/typing_surface/mod.rs

@@ -27,6 +27,7 @@ pub mod insert_form_c;
 pub mod update_with_where;
 pub mod update_all_rows;
 pub mod delete_with_where;
+pub mod where_expression;
 pub mod delete_all_rows;
 pub mod create_table;
 pub mod drop_column;