Walker + parser: surface mid-typing after separators and Form C/A ambiguity

The typing-surface matrix exposed two bugs the existing 859-test suite
missed:

walk_repeated: when the separator consumed but the inner item failed
at EOF, the old path rolled the separator back and reported a definite
error at the rollback position (`insert into T (a, ` flashed red on
the `,` after each comma). Now propagates Incomplete with the inner's
expected set so the input renderer treats it as mid-typing.

build_insert Form C path: `insert into T (col)` walked to a complete
match but produced `values: []` because Form C's value collector drops
ident-shaped items. The user almost certainly meant Form A and just
hasn't typed `values (...)` yet. Reject with a ValidationError naming
the Form-A continuation; classify_input now reports IncompleteAtEof.

completion_probe / expected_at_input: ValidationFailed used to return
an empty expected set, leaving Tab with nothing to offer at the new
Form-A flag point. Now surface result.tail_expected (skipped-Optional
expectations captured before validation fired) so `values` is still
offered as a candidate.

src/dsl/walker/mod.rs

@@ -271,7 +271,14 @@ pub fn completion_probe(
         outcome::WalkOutcome::Match { .. } => result.tail_expected,
         outcome::WalkOutcome::Incomplete { expected, .. }
         | outcome::WalkOutcome::Mismatch { expected, .. } => expected,
-        outcome::WalkOutcome::ValidationFailed { .. } => Vec::new(),
+        // Validation failure path: the walker matched the
+        // structural shape but the AST builder rejected (e.g.
+        // Form C with column-shaped items). The walker still
+        // captured the skipped-Optional expectations before the
+        // validation fired — surface those so the user gets
+        // useful Tab candidates even at a validation-flagged
+        // position.
+        outcome::WalkOutcome::ValidationFailed { .. } => result.tail_expected,
     };
     CompletionProbe {
         expected,
@@ -324,7 +331,14 @@ pub fn expected_at_input(source: &str) -> Vec<outcome::Expectation> {
         outcome::WalkOutcome::Match { .. } => result.tail_expected,
         outcome::WalkOutcome::Incomplete { expected, .. }
         | outcome::WalkOutcome::Mismatch { expected, .. } => expected,
-        outcome::WalkOutcome::ValidationFailed { .. } => Vec::new(),
+        // Validation failure path: the walker matched the
+        // structural shape but the AST builder rejected (e.g.
+        // Form C with column-shaped items). The walker still
+        // captured the skipped-Optional expectations before the
+        // validation fired — surface those so the user gets
+        // useful Tab candidates even at a validation-flagged
+        // position.
+        outcome::WalkOutcome::ValidationFailed { .. } => result.tail_expected,
     }
 }