walker: expression WARNING diagnostics (ADR-0027 step C, folds ADR-0026 §7)

Type-mismatched comparisons and `= NULL` / `!= NULL` in a
WHERE expression now yield WARNING diagnostics — the command
still parses and runs (the ADR-0026 §7 permissive posture is
unchanged), but the validity indicator can flag it before
submission.

Computed post-walk from the built command's `Expr` against
the table's column types: a Compare / Between / In with a
column operand and a non-null literal whose type the column
cannot hold, or a Compare with `=` / `!=` against NULL. New
catalog keys `diagnostic.type_mismatch` / `diagnostic.eq_null`.

This is ADR-0026's deferred step 5, folded into ADR-0027's
diagnostics-severity model as the user requested.

src/friendly/keys.rs

@@ -38,6 +38,8 @@
 /// `(key, expected_placeholders)`. Sorted by key for grep-ability.
 pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     // ---- Pre-submit diagnostics (ADR-0027) ----
+    ("diagnostic.eq_null", &[]),
+    ("diagnostic.type_mismatch", &["column", "type"]),
     ("diagnostic.unknown_column", &["name", "table"]),
     ("diagnostic.unknown_table", &["name"]),
     // ---- Already-exists collisions (anchor: "already exists") ----