walker: diagnostics-severity model + input_verdict (ADR-0027 step A)

Adds `Severity` (Error / Warning, ordered so Error > Warning)
and `Diagnostic { severity, span, message }` in
`walker::outcome`, plus a `diagnostics` field on `WalkResult`
— the schema-aware findings layered on a structurally-valid
parse (ADR-0027 §2).

`input_verdict(source, schema)` is the validity-indicator
entry point: `None` when the input would run clean (and for
empty input), `Some(Error)` for a parse failure or unknown
command, `Some(Warning)` for the ADR-0026 expression flags.
The verdict is the highest severity across the parse outcome
and the diagnostics set.

`diagnostics` is empty at this step — the schema-existence
(ERROR) and expression (WARNING) passes that fill it land
next. Covered by `input_verdict` unit tests.

src/dsl/walker/outcome.rs

@@ -159,11 +159,52 @@ pub struct ByteClass {
     pub class: HighlightClass,
 }
 
+/// Severity of a pre-submit [`Diagnostic`] (ADR-0027 §1).
+///
+/// Ordered so `Error > Warning` — taking the `max` across a
+/// diagnostic set gives the validity indicator's verdict.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub enum Severity {
+    /// Valid and runnable, but very likely not what the user
+    /// intends — a type-mismatched comparison, `= NULL`
+    /// (ADR-0026 §7).
+    Warning,
+    /// Known to fail if submitted now — a parse error, or a
+    /// reference to a table / column that does not exist.
+    Error,
+}
+
+/// A problem the walker found in the input *before* submission
+/// (ADR-0027 §2). The severity drives the validity indicator;
+/// the span drives highlighting; the message is the hint text.
+///
+/// Note: the parse outcome (`Incomplete` / `Mismatch` /
+/// `ValidationFailed`) is *not* recorded as a `Diagnostic` —
+/// the indicator reads it from `WalkOutcome` directly
+/// (ADR-0027 §2, "the highest severity across the outcome and
+/// the diagnostics"). `diagnostics` carries the schema-aware
+/// findings layered on top of a structurally-valid parse.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Diagnostic {
+    pub severity: Severity,
+    /// Byte range in the source the diagnostic refers to.
+    pub span: (usize, usize),
+    /// Ready-to-show message — already catalog-translated by
+    /// whichever pass produced the diagnostic.
+    pub message: String,
+}
+
 #[derive(Debug, Clone)]
 pub struct WalkResult {
     pub outcome: WalkOutcome,
     pub matched_path: MatchedPath,
     pub per_byte_class: Vec<ByteClass>,
+    /// Schema-aware pre-submit findings layered on a
+    /// structurally-valid parse (ADR-0027): unknown table /
+    /// column references (ERROR), type-mismatched WHERE
+    /// comparisons and `= NULL` (WARNING). Empty when the
+    /// input does not parse — those are the outcome's job.
+    pub diagnostics: Vec<Diagnostic>,
     /// Optional-Optional expectations the walker could have
     /// accepted but didn't because the outer shape ran out at a
     /// node boundary (ADR-0024 §architecture, round-5 follow-up).