Fix H3: field-disc union wire convention — shared-then-variant (review #006)

Decision (recorded as an ADR-011 addendum): the packed-mode wire layout
for a field-name-discriminator TUnion is shared-then-variant — the
union's declared `fields` (disc + shared fields) first, then the
variant's own fields. Reader and materializer already implemented this;
LayoutBuilder was corrected from variant-only layout.

Enforcement in BastUnion::parse (the choke point every consumer
inherits — union roots at BastDoc::new, referenced unions at
resolve_ref):
- discriminator field must be declared in `fields`
- `fields` must not contain duplicate names
- variants must not re-declare shared fields (checked inline and
  through $ref resolution — parse chain now threads the doc root)
- the discriminator field must be the FIRST entry in `fields` (the
  reader reads the disc at the union start; a later position made it
  dispatch on the wrong bytes — H3 item 2, probe-verified)

Schemas relying on the old variant-only builder convention (variants
re-declaring shared fields) are rejected with a clean Schema error
naming the convention. Breaking for 0.2.0-era re-declaring schemas;
announced with 0.3.x.

- L5: FieldValue::Union::variant_start doc now states per-kind
  semantics (byte-disc: union_start + disc.offset + disc.size;
  field-disc: after the shared walk).
- L6: roundtrip test added (poc_roundtrip.rs) — LayoutBuilder write →
  SequentialReader read → materialize_packed → validate_bytes over a
  field-disc union with a second shared field and non-redeclaring
  variant; pins event.type@0/seq@1/handle@5, total 10.
- ADR-011: Status-block addendum recording the convention decision,
  the no-re-declare rule, and the breaking-constraint note.
- Review #006 updated: H3/L5/L6 resolution blocks, resolution log,
  recommended order.

Verified: 488 tests green (410+17+34+15+12, 2 pre-existing ignored),
clippy -D warnings clean, wasm32 build green, cargo doc zero warnings.

docs/architecture/decisions/011-compiled-read-plan-for-packed-mode.md

@@ -24,6 +24,31 @@ are pre-implementation refinements to types that do not yet exist on
 `main`; the ADR-011 decision (a compiled `ReadPlan` for packed reads)
 is unchanged.
 
+**Addendum — field-disc union wire convention (2026-09-02, review #006
+H3):** the packed-mode wire layout for a field-name-discriminator
+TUnion is **shared-then-variant**: the union's declared `fields` (the
+discriminator field + any shared fields) occupy the union's start
+offset in declaration order, and the selected variant's fields follow
+immediately after all shared fields. All three packed-mode consumers
+now implement this one convention: the reader and materializer already
+walked `shared` then the variant (the `shared` sub-plan shape above);
+`LayoutBuilder` was corrected in the same pass — it previously laid out
+only the selected variant, disagreeing with the read side on span and
+field positions (review #006 H3 item 1). The convention requires that
+a variant **must not re-declare** the discriminator field or any
+shared field — `BastUnion::parse` enforces this at parse time (also:
+the discriminator field must be declared in `fields`, and `fields`
+must not contain duplicate names), so the shared walk and the variant
+walk cover disjoint fields and the wire has exactly one copy of each
+shared byte. Schemas whose variants redeclared shared fields were
+ambiguous under the old split-convention behavior and are rejected
+rather than given a silent meaning; this is a **breaking wire-format
+constraint** for any 0.2.0-era schema that relied on re-declaration,
+announced with the 0.3.x series. `DiscriminatorPlan::Field`'s disc
+read is at the disc field's position within the shared walk (the
+materializer's position-correct behavior, review #006 H3 item 2); the
+reader's plan-walk reads it there too.
+
 ## Context
 
 Review #004 (`docs/reviews/004-performance-review.md`) measured the