LayoutBuilder caches the owned BastDoc (ADR-012 §2a, plan phase 4)

The builder stores doc: BastDoc + endian (the doc_value: Value +
root_name: String cache is gone); new parses the typed tree once and
build walks &self.doc — the per-build BastDoc::new re-parse
(layout_builder.rs M1) is retired.

- build's root-is-struct re-check replaces its unreachable!() with a
  clean Schema error (AGENTS.md §3 never-panic; invariant unchanged —
  new already rejects non-struct roots).
- Boxing fallout: the builder now holds the full owned tree, so
  Layout::Packed boxes it (Box<LayoutBuilder>) to keep the engine's
  Layout enum variant sizes balanced (clippy large_enum_variant).
  layout_builder() still returns Option<&LayoutBuilder> via
  auto-deref; public API unchanged.

Verification: 465 tests pass unchanged (layout_builder.rs suites
drive new/build through the public API); clippy -D warnings clean;
wasm32 release build green.

src/engine.rs

@@ -54,7 +54,7 @@ enum Layout {
     /// hands out (ADR-007 — the reader owns its cursor state, so the
     /// engine is a factory, not a holder).
     Packed {
-        builder: LayoutBuilder,
+        builder: Box<LayoutBuilder>,
         plan: Arc<ReadPlan>,
     },
     /// Aligned static layout. Field offsets are precomputed in an
@@ -152,7 +152,7 @@ impl AlkTypeEngine {
         let validation_plan = Arc::new(ValidationPlan::compile(&doc)?);
         let layout = match mode {
             LayoutMode::Packed => {
-                let builder = LayoutBuilder::new(bast_doc, root_name)?;
+                let builder = Box::new(LayoutBuilder::new(bast_doc, root_name)?);
                 let plan = Arc::new(ReadPlan::compile(bast_doc, root_name)?);
                 Layout::Packed { builder, plan }
             }

src/layout_builder.rs

@@ -129,8 +129,7 @@ impl PackedLayout {
 /// this is correct for protocol wire formats, which pack fields tightly.
 #[derive(Debug)]
 pub struct LayoutBuilder {
-    doc_value: Value,
-    root_name: String,
+    doc: BastDoc,
     endian: Endian,
 }
 
@@ -140,9 +139,9 @@ impl LayoutBuilder {
     /// The root type must be a struct. Endianness is read from the
     /// root struct's `endian` annotation (defaults to little-endian).
     ///
-    /// The builder retains the raw BAST `Value` and re-parses the typed
-    /// tree on each [`build`](Self::build) call; this is cheap (the
-    /// typed tree borrows from the source without cloning field data).
+    /// The builder parses the owned [`BastDoc`] once here (ADR-012 §2a)
+    /// and every [`build`](Self::build) call reuses the cached tree —
+    /// no re-parse, no `Value` clone per build.
     ///
     /// # Errors
     ///
@@ -161,11 +160,7 @@ impl LayoutBuilder {
             }
         };
         let endian = struct_node.endian();
-        Ok(Self {
-            doc_value: bast_doc.clone(),
-            root_name: root_name.to_string(),
-            endian,
-        })
+        Ok(Self { doc, endian })
     }
 
     /// Build the packed layout given actual data sizes for variable-length
@@ -187,14 +182,17 @@ impl LayoutBuilder {
     ///   sizes in `var_sizes`, missing discriminator values, or unknown
     ///   discriminator values.
     pub fn build(&self, var_sizes: &HashMap<String, usize>) -> Result<PackedLayout, AlkTypeError> {
-        let doc = BastDoc::new(&self.doc_value, &self.root_name)?;
-        let root_def = doc.root_def();
+        let root_def = self.doc.root_def();
         let struct_node = match root_def.kind() {
             BastDefKind::Struct(s) => s,
-            _ => unreachable!("checked in new; doc_value is immutable"),
+            _ => {
+                return Err(AlkTypeError::Schema(
+                    "internal: LayoutBuilder root is not a struct (checked in new)".to_string(),
+                ));
+            }
         };
         let mut ctx = BuildCtx {
-            doc: &doc,
+            doc: &self.doc,
             var_sizes,
             fields: Vec::new(),
         };