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/friendly/keys.rs

@@ -158,6 +158,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("parse.custom.bind_type_mismatch", &["found", "expected"]),
     ("parse.custom.change_column_flags_exclusive", &[]),
     ("parse.custom.create_table_needs_pk", &[]),
+    ("parse.custom.insert_form_a_missing_values", &["columns"]),
     ("parse.custom.on_action_specified_twice", &["target"]),
     ("parse.custom.replay_path_expected", &[]),
     ("parse.custom.unknown_action", &["found", "expected"]),