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/grammar/data.rs

@@ -422,6 +422,36 @@ fn build_insert(path: &MatchedPath) -> Result<Command, ValidationError> {
         // Form C: the first paren contained the value list. The
         // Repeated tagged the matched values via their natural
         // MatchedKind (Word/NumberLit/StringLit); collect them.
+        //
+        // Form-A-without-`values` recovery: the shared
+        // INSERT_PAREN_ITEM choice accepts both VALUE_LITERAL
+        // and Ident{Columns} so that Form A can resolve
+        // column-name items inside its `( cols )` list. When the
+        // user types `insert into T (col)` (column-shaped item,
+        // no `values` keyword), the grammar walks to a complete
+        // match but the user almost certainly meant Form A and
+        // forgot the `values (...)` suffix. Reject here with a
+        // ValidationError — the walker classifies validation
+        // errors as `at_eof: true`, so the input renderer
+        // surfaces this as IncompleteAtEof (mid-typing) rather
+        // than dispatching a logically-broken Form C insert with
+        // an empty value list.
+        let user_listed_columns: Vec<String> = path
+            .items
+            .iter()
+            .filter_map(|i| match &i.kind {
+                MatchedKind::Ident {
+                    role: "insert_first_item",
+                } => Some(i.text.clone()),
+                _ => None,
+            })
+            .collect();
+        if !user_listed_columns.is_empty() {
+            return Err(ValidationError {
+                message_key: "parse.custom.insert_form_a_missing_values",
+                args: vec![("columns", user_listed_columns.join(", "))],
+            });
+        }
         let values = collect_values_in_parens(path, first_paren_idx)?;
         Ok(Command::Insert {
             table,

src/dsl/walker/driver.rs

@@ -497,6 +497,12 @@ fn walk_repeated(
     loop {
         let saved_path_len = path.items.len();
         let saved_byte_len = per_byte.len();
+        // Track whether the separator successfully consumed
+        // before the inner attempt. Used below to distinguish
+        // "user typed `,` then stopped at EOF — mid-typing the
+        // next item" from "list naturally ended at the inner
+        // boundary".
+        let mut sep_consumed_to: Option<usize> = None;
         let result = if count == 0 {
             walk_node(source, cur, inner, ctx, path, per_byte)
         } else if let Some(sep) = separator {
@@ -504,6 +510,7 @@ fn walk_repeated(
             let sep_saved_byte = per_byte.len();
             match walk_node(source, cur, sep, ctx, path, per_byte) {
                 NodeWalkResult::Matched { end, .. } => {
+                    sep_consumed_to = Some(end);
                     walk_node(source, end, inner, ctx, path, per_byte)
                 }
                 NodeWalkResult::NoMatch { .. } => {
@@ -521,7 +528,31 @@ fn walk_repeated(
                 cur = end;
                 count += 1;
             }
-            NodeWalkResult::NoMatch { expected, .. } => {
+            NodeWalkResult::NoMatch { expected, position: inner_pos } => {
+                // Mid-typing-the-next-item recovery: if the
+                // separator just consumed and the inner failed
+                // at EOF, the user is partway through typing the
+                // next item — propagate as Incomplete so the
+                // outer walker classifies the input as
+                // mid-typing rather than rolling the separator
+                // back and producing a structural Mismatch at
+                // the separator position.
+                //
+                // Without this branch, `insert into T (a, ` at
+                // EOF would roll back the `,`, then the outer
+                // `(`-list expected `)` at `cur`, see the
+                // separator instead, and report a definite
+                // error at the separator. Real users hit this
+                // every time they type a comma and pause.
+                if let Some(post_sep) = sep_consumed_to {
+                    let post_ws = skip_whitespace(source, post_sep);
+                    if post_ws >= source.len() {
+                        return NodeWalkResult::Incomplete {
+                            position: inner_pos,
+                            expected,
+                        };
+                    }
+                }
                 path.items.truncate(saved_path_len);
                 per_byte.truncate(saved_byte_len);
                 last_expected = Some(expected);

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,
     }
 }