Make BastDoc owned: drop Bast* lifetimes (ADR-012 §2a, plan phase 3)

Every Bast* type drops <'a>: &'a str -> String, &'a Value -> Value
(deferred decision 1: plain String/Value — the tree is built once;
Arc<str> name-sharing needs a bench justification that doesn't exist).
BastDoc::new(&Value, &str) still takes references in and clones into
owned storage; the doc gains Clone. resolve_ref/resolve_typeref/
resolve_typeref_as_def return owned types.

- Engine ownership flip: AlkTypeEngine holds the owned BastDoc
  (replacing bast_doc: Value + root_name: String; root_name()
  delegates to the doc), killing its three per-call BastDoc::new
  re-parses (aligned validate_bytes, read_field, write_field — the
  review #004 M1 pattern removed by construction; phase 5 retires the
  lookup_leaf_field walk itself). New public accessor root_name()
  (additive). Engine Send + Sync with the owned doc, asserted in the
  existing thread-share test.
- Bonus cleanup: materialize_typeref_packed's dead _field param
  dropped (phase 2 left it dangling). Under ownership, keeping it
  would force a deep Value clone per array element / record value /
  union variant via dummy_field_for. The param, dummy_field_for, and
  ty_source are gone; no behavior change (the arg was already
  ignored). BastField::synthetic keeps an owned-signature
  #[allow(dead_code)] definition (no remaining callers today).
- Consumers adapted: OffsetMap::compute(&BastDoc),
  materialize_aligned(&BastDoc, ...) (no lifetime), BuildCtx/
  ComputeCtx hold &'d BastDoc, tunion/discriminator name borrows,
  lib.rs module doc. LayoutBuilder's doc_value re-parse cache is
  unchanged pending phase 4.

Verification: 465 tests pass with zero test-logic changes (bast.rs
suites exercise every parser path through the public API); clippy
-D warnings clean; cargo doc zero warnings; wasm32 release build
green.

tests/tunion_dispatch.rs

@@ -49,7 +49,7 @@ fn sftp_like_byte_union_doc() -> serde_json::Value {
     })
 }
 
-fn union_of<'a>(root: &'a serde_json::Value, name: &'a str) -> BastUnion<'a> {
+fn union_of(root: &serde_json::Value, name: &str) -> BastUnion {
     let doc = BastDoc::new(root, name).expect("bast doc");
     match doc.root_def().kind() {
         BastDefKind::Union(u) => u.clone(),