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.

docs/plans/030-compiled-forms.md

@@ -482,7 +482,20 @@ wasm-relevant).
 
 ---
 
-## Phase 4 — `LayoutBuilder` caches the owned `BastDoc` (ADR-012 §2a)
+## Phase 4 — `LayoutBuilder` caches the owned `BastDoc` (ADR-012 §2a) — **DONE (2026-09-02)**
+
+> **Status: implemented.** `LayoutBuilder` stores `doc: BastDoc` +
+> `endian` (the `doc_value: Value` + `root_name: String` cache is
+> gone); `new` parses once, `build` walks `&self.doc` — the
+> `layout_builder.rs` re-parse (M1) is retired. The `build`-time
+> root-is-struct re-check replaced its `unreachable!()` with a clean
+> `Schema` error (AGENTS.md §3 never-panic; the invariant is
+> unchanged — `new` already rejects non-struct roots). Boxing fallout:
+> the builder now holds the full owned tree, so `Layout::Packed`
+> boxes it (`builder: 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.
 
 **Goal:** `LayoutBuilder::new` parses the owned `BastDoc` once and
 stores it; `build` reuses it. Removes the `layout_builder.rs:190`