fix(io): enforce category limit in import wizard

The import wizard now proactively checks the `MAX_CATEGORIES` limit during
the proposal and configuration steps.

Advancing is blocked with a descriptive message if the limit would be
exceeded.

Fixed UI rendering order in `ImportWizardWidget` so messages are correctly
displayed.

Add regression tests for category limit enforcement.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)

crates/improvise-io/src/import/wizard.rs

@@ -9,6 +9,11 @@ use crate::formula::parse_formula;
 use crate::model::cell::{CellKey, CellValue};
 use crate::workbook::Workbook;
 
+/// Mirrors the model's private `MAX_CATEGORIES` limit (regular categories
+/// only; virtual `_Index`/`_Dim`/`_Measure` don't count). Used to pre-check
+/// proposals before the wizard's confirm step (improvise-mzv).
+const MAX_CATEGORIES: usize = 12;
+
 // ── Pipeline (no UI state) ────────────────────────────────────────────────────
 
 /// Pure data + logic for turning a JSON value into a Model.
@@ -80,6 +85,22 @@ impl ImportPipeline {
         }
     }
 
+    /// Number of regular categories `build_model` would create: accepted
+    /// Category/TimeCategory fields plus derived date-component categories.
+    /// (Measures and labels map to non-regular categories and don't count.)
+    pub fn proposed_category_count(&self) -> usize {
+        self.proposals
+            .iter()
+            .filter(|p| p.accepted)
+            .map(|p| match p.kind {
+                FieldKind::Category => 1,
+                FieldKind::TimeCategory if p.date_format.is_some() => 1 + p.date_components.len(),
+                FieldKind::TimeCategory => 1,
+                FieldKind::Measure | FieldKind::Label => 0,
+            })
+            .sum()
+    }
+
     /// Build a Workbook from the current proposals. Pure — no side effects.
     pub fn build_model(&self) -> Result<Workbook> {
         let categories: Vec<&FieldProposal> = self
@@ -297,6 +318,17 @@ impl ImportWizard {
     // ── Step transitions ──────────────────────────────────────────────────────
 
     pub fn advance(&mut self) {
+        // Pre-check the category limit where the user can still fix it
+        // (improvise-mzv): block here instead of failing in build_model()
+        // after the final confirm.
+        if matches!(
+            self.step,
+            WizardStep::ReviewProposals | WizardStep::ConfigureDates
+        ) && let Some(msg) = self.category_limit_message()
+        {
+            self.message = Some(msg);
+            return;
+        }
         self.step = match self.step {
             WizardStep::Preview => {
                 if self.pipeline.array_paths.len() > 1 && self.pipeline.needs_path_selection() {
@@ -322,6 +354,18 @@ impl ImportWizard {
         self.message = None;
     }
 
+    /// User-facing message when the accepted proposals would exceed the
+    /// model's regular-category limit; `None` when within the limit.
+    fn category_limit_message(&self) -> Option<String> {
+        let count = self.pipeline.proposed_category_count();
+        (count > MAX_CATEGORIES).then(|| {
+            format!(
+                "Too many categories: {count} proposed, max {MAX_CATEGORIES}. \
+                 Mark fields as Measure (c) or toggle them off (Space)."
+            )
+        })
+    }
+
     fn has_time_categories(&self) -> bool {
         self.pipeline
             .proposals
@@ -856,6 +900,73 @@ mod tests {
         assert!(!w.pipeline.records.is_empty());
     }
 
+    // ── Category limit pre-check ────────────────────────────────────────
+
+    /// Build records whose fields `c00..cNN` are all low-cardinality strings,
+    /// so the analyzer proposes every field as an accepted Category.
+    fn category_records(n_fields: usize) -> serde_json::Value {
+        let recs: Vec<serde_json::Value> = (0..3)
+            .map(|r| {
+                let mut m = serde_json::Map::new();
+                for i in 0..n_fields {
+                    m.insert(format!("c{i:02}"), json!(format!("v{r}")));
+                }
+                serde_json::Value::Object(m)
+            })
+            .collect();
+        serde_json::Value::Array(recs)
+    }
+
+    /// BUG (improvise-mzv): with more than MAX_CATEGORIES (12) accepted
+    /// Category proposals, the wizard let the user advance through every
+    /// remaining step; build_model() only failed at the final confirm.
+    /// Advancing past ReviewProposals must be blocked with a message telling
+    /// the user to mark fields as Measure or toggle them off.
+    #[test]
+    fn wizard_blocks_advance_when_over_category_limit() {
+        let mut w = ImportWizard::new(category_records(13));
+        assert_eq!(w.step, WizardStep::ReviewProposals);
+        assert_eq!(
+            w.pipeline
+                .proposals
+                .iter()
+                .filter(|p| p.accepted && p.kind == FieldKind::Category)
+                .count(),
+            13,
+            "sanity: all 13 fields proposed as accepted categories"
+        );
+
+        w.advance();
+        assert_eq!(
+            w.step,
+            WizardStep::ReviewProposals,
+            "advance must be blocked while over the category limit"
+        );
+        let msg = w.message.as_deref().expect("a message explaining the block");
+        assert!(msg.contains("Measure"), "message should tell the user how to fix it: {msg}");
+    }
+
+    #[test]
+    fn wizard_allows_advance_at_exactly_category_limit() {
+        let mut w = ImportWizard::new(category_records(12));
+        assert_eq!(w.step, WizardStep::ReviewProposals);
+        w.advance();
+        assert_ne!(w.step, WizardStep::ReviewProposals);
+    }
+
+    #[test]
+    fn wizard_advance_unblocks_after_marking_field_as_measure() {
+        let mut w = ImportWizard::new(category_records(13));
+        w.advance();
+        assert_eq!(w.step, WizardStep::ReviewProposals);
+
+        // Toggle one field off; the wizard should now let the user through.
+        w.cursor = 0;
+        w.toggle_proposal();
+        w.advance();
+        assert_ne!(w.step, WizardStep::ReviewProposals);
+    }
+
     // ── Formula editing in wizard ───────────────────────────────────────
 
     #[test]