ui: overlay diagnostic spans on the input field (ADR-0027 §2)

render_input_runs now overlays the walker's schema-aware
diagnostics: an unknown table/column ERROR is recoloured
tok_error, an expression WARNING (type mismatch, = NULL, LIKE
on a numeric column) recoloured theme.warning. New overlay_span
covers a token's whole byte range (overlay_error only hits the
run at a single byte). New walker::input_diagnostics is the
shared entry point.

The overlay is global — every flagged token is coloured
wherever it sits, not only under the cursor — which is exactly
ADR-0027's motivation. The existing cursor-local invalid-ident
overlay is kept (it covers in-progress idents diagnostics do
not); the two are additive and idempotent.

5 input_render tests (unknown table/column, type-mismatch
literal precise, LIKE-on-numeric, clean command). 1113 passing,
clippy clean.

src/dsl/walker/mod.rs

@@ -332,6 +332,34 @@ pub fn input_verdict(
     outcome_severity.into_iter().chain(diag_severity).max()
 }
 
+/// The schema-aware diagnostics for `source` (ADR-0027 §2).
+///
+/// Schema-existence ERRORs (unknown table / column) and
+/// expression WARNINGs. The highlight overlay and the hint
+/// panel both read these for *where* and *why*; the indicator
+/// ([`input_verdict`]) is the severity summary over them.
+///
+/// Empty for empty input, an unrecognised command, or a parse
+/// that never reached a structural `Match` — a parse failure
+/// carries its own ERROR through the outcome, not through a
+/// `Diagnostic`, and is highlighted by the existing
+/// definite-error path.
+#[must_use]
+pub fn input_diagnostics(
+    source: &str,
+    schema: Option<&crate::completion::SchemaCache>,
+) -> Vec<outcome::Diagnostic> {
+    if source.trim().is_empty() {
+        return Vec::new();
+    }
+    let mut ctx = schema.map_or_else(
+        context::WalkContext::new,
+        context::WalkContext::with_schema,
+    );
+    let (result, _cmd) = walk(source, outcome::WalkBound::EndOfInput, &mut ctx);
+    result.map_or_else(Vec::new, |r| r.diagnostics)
+}
+
 /// Schema-existence diagnostics (ADR-0027 §2).
 ///
 /// A matched `IdentSource::Tables` token whose name is not in