feat(hint): advertise the optional seed count in the hint panel (#26)

At `seed <table> ▮` the hint showed only the `set`/`--seed` chips and
never mentioned the optional row count — a bare positional number with no
candidate, on an already-complete command, so neither the candidate
ladder nor the resolver surfaced it. (A prior IntroProse attempt was
reverted: pending_hint_mode is cleared by the trailing optionals.)

Carry a skipped Optional's IntroProse hint: walk_optional stashes the
inner's key into a new WalkContext.surviving_intro_hint (key + position)
before the empty match clears pending_hint_mode; the snapshot keeps it
only when the skip position is the cursor (so it never leaks past a
later-consumed `set …` clause, nor once the count is given); the
resolver returns it ahead of the empty-expected short-circuit. The seed
count is wrapped Hinted{IntroProse("hint.seed_count")}; the prose names
the count (default 20), the `.column` column-fill form, and `set` /
`--seed`. Tab still cycles the keywords.

Only IntroProse is carried; ProseOnly/ForceProse and the CREATE-TABLE
element (a required Repeated) are untouched. No AmbientHint/renderer
change. Fires in both modes.

ADR-0022 Amendment 7; +3 tests.

src/dsl/walker/context.rs

@@ -134,6 +134,17 @@ pub struct WalkContext<'a> {
     /// resolver reads this directly instead of inferring the
     /// slot kind from the shape of the expected set.
     pub pending_hint_mode: Option<crate::dsl::grammar::HintMode>,
+    /// An `IntroProse` hint captured from an *optional* slot that
+    /// the walk skipped (issue #26). Unlike `pending_hint_mode`
+    /// (cleared on the very next match — including the empty match
+    /// of a skipped `Optional`), this survives the trailing
+    /// optional siblings so the hint reaches the resolver for a
+    /// position like `seed <table> ▮`, where the optional row
+    /// count is otherwise invisible. Carries the catalog key and
+    /// the byte position the optional was skipped at; the resolver
+    /// uses it only when that position is the cursor (so it doesn't
+    /// leak past a later-consumed clause).
+    pub surviving_intro_hint: Option<(&'static str, usize)>,
     /// The columns the user explicitly listed in
     /// `insert into <T> (col1, col2, …) values (…)` (Form A),
     /// in declaration order.
@@ -232,6 +243,7 @@ impl<'a> WalkContext<'a> {
             pending_value_type: None,
             pending_value_column: None,
             pending_hint_mode: None,
+            surviving_intro_hint: None,
             user_listed_columns: None,
             subgrammar_depth: 0,
             from_scope_stack: vec![ScopeFrame::default()],
@@ -254,6 +266,7 @@ impl<'a> WalkContext<'a> {
             pending_value_type: None,
             pending_value_column: None,
             pending_hint_mode: None,
+            surviving_intro_hint: None,
             user_listed_columns: None,
             subgrammar_depth: 0,
             from_scope_stack: vec![ScopeFrame::default()],