Hint: surface ( as a branching candidate; stop red-flagging in-progress Form A values

Two related fixes from a user-reported snag:

1. After typing \`insert into Orders \`, the hint suggested only
   \`values\` even though the user could also choose \`(\` to
   open Form A (the explicit-column-list variant). The walker
   reports both \`Expectation::Word("values")\` and
   \`Expectation::Punct('(')\` at that position, but
   \`candidates_at_cursor\` had a blanket "no punctuation as Tab
   candidate" policy.

   Loosened the policy to surface branching punct
   (specifically \`(\` opening a sub-shape). Closing punct
   (\`)\`), separators (\`,\`), and content-trailing punct (\`:\`,
   \`=\`, \`.\`) stay out — the user types those naturally and
   advertising them in the Tab menu is noise. New
   \`CandidateKind::Punct\` so the renderer colors it as punct
   rather than mis-classifying as a keyword.

2. While typing \`insert into Orders (id, CustId, Total) values
   (42, 89, 17.59\` (no closing paren yet), the word \`values\`
   was rendered in \`tok_error\` red. The walker's
   \`Optional(Seq[values, '(', list, ')'])\` was rolling back on
   the partial inner match — treating \`(id, CustId, Total)\` as
   Form C (bare value list) followed by trailing junk starting
   at \`values\`. The classify_input call thus returned
   \`DefiniteErrorAt(<values byte>)\` and the renderer overlaid.

   Tightened \`walk_optional\`: roll back only when the inner
   reports NoMatch (or Incomplete / Mismatch without consuming
   anything). Once the inner has committed to at least one
   terminal (e.g. matched the \`values\` keyword), propagate
   Incomplete / Mismatch up — the user is mid-typing the
   optional's content and rolling back would lose their
   intent.

   The pre-existing chumsky-or_not-style aggressive rollback
   covered cases like \`save Customers\` (Optional(\`as\`)
   inner is a single Word that returns NoMatch without
   consuming, so rollback still fires). Those keep working.

3. Side effect: with \`Optional\` no longer hiding the
   in-progress Form A from the leading slice, the walker on
   \`create table T with \` correctly reports the next-expected
   keyword as \`pk\` — so cursor at the end of the complete
   command \`create table T with pk\` would now re-offer \`pk\`
   as a Tab candidate against the partial \"pk\". Added a final
   filter: when the full input is a valid parse AND the
   partial prefix is non-empty, drop candidates that equal the
   partial exactly. Preserves schema narrowing
   (\`show data Cu\` → \`Customers\` is not an exact match).

Tests:
- New \`in_progress_form_a_values_list_classifies_as_incomplete\`
  asserts the input-state for the user's exact scenario.
- New \`open_paren_branching_punct_surfaces_after_insert_into_table\`
  and \`open_paren_candidate_is_classified_as_punct_kind\` cover
  the punct-as-candidate surface.
- Renamed and rewrote \`punctuation_expected_does_not_produce_candidates\`
  to \`non_branching_punctuation_is_not_surfaced_as_candidate\`
  to document the new finer-grained policy.
- Existing tests for \`save Tab → as\` and the schema-
  narrowing case continue to pass.

Tests: 854 passing, 0 failing, 1 ignored. Clippy clean.

src/ui.rs

@@ -766,6 +766,7 @@ fn render_candidate_line(
             crate::completion::CandidateKind::Keyword => theme.tok_keyword,
             crate::completion::CandidateKind::Identifier => theme.tok_identifier,
             crate::completion::CandidateKind::Flag => theme.tok_flag,
+            crate::completion::CandidateKind::Punct => theme.tok_punct,
         };
         let mut s = Style::default().fg(base_fg);
         if Some(i) == selected {