Centralize scattered OQs into the tracker (OQ-004 through OQ-007)

Four open questions were scattered inline in builder.md and the POC
findings doc. Moved them into the central OQ tracker under
docs/architecture/questions/ and updated the index:

- OQ-004: Discriminator::Field name type (&str vs String) — raised in
  builder.md during ADR-009 spec drafting
- OQ-005: Union materialization shape (byte-offset vs field-name
  consistency) — raised in the POC findings
- OQ-006: Builder spec Example 3 wrap Union in Struct — raised in the
  POC findings (doc fix; engine requires AlkType:Struct at top level)
- OQ-007: Bytes materialization lossy UTF-8 — raised in the POC
  findings (blocks SFTP use case for validate_bytes)

Index updates:
- open-questions.md: new 'Schema Construction' and 'Validation' theme
  groups; new 'Open' section for active investigation targets (distinct
  from 'Deferred / Blocked' which holds scope-parked OQs)
- README.md: OQ table extended with OQ-004 through OQ-007
- builder.md: inline OQ-004 replaced with a tracker reference
- findings.md: inline OQ-005/006/007 replaced with tracker references

Doc-only change; 369 tests pass.

docs/research/alktype-builder-poc/findings.md

@@ -109,7 +109,8 @@ Either:
 - Document the constraint explicitly ("the engine requires a top-level
   `AlkType:Struct`; a `Union` is a field type within a struct").
 
-This is a documentation fix, not an implementation change.
+This is a documentation fix, not an implementation change. Tracked as
+[OQ-006](../../../architecture/questions/006-builder-spec-example-3-wrap-union.md).
 
 ### 2. Builder field order is preserved as required
 
@@ -200,13 +201,16 @@ validator for JSON-only schemas".
    fields including the discriminator. The byte-offset discriminator
    pattern returns a `__discriminator`-tagged object. This asymmetry
    needs resolution before shipping (likely both should return the same
-   shape — see Open Questions).
+   shape — tracked as
+   [OQ-005](../../../architecture/questions/005-union-materialization-shape.md)).
 3. **`Bytes` materialization** uses `String::from_utf8_lossy` to convert
    raw bytes to a JSON string (the validator expects a string for
    `AlkType:Bytes` — see schema-layer.md §TBytes). This is lossy for
    non-UTF-8 bytes; strict byte-preserving validation would need a
    different validator form. For the chunk header (no `Bytes` fields),
-   this doesn't matter, but it's a follow-up for the SFTP use case.
+   this doesn't matter, but it's a follow-up for the SFTP use case
+   (tracked as
+   [OQ-007](../../../architecture/questions/007-bytes-materialization-lossy-utf8.md)).
 4. **`encoding` setter** in the builder only handles the
    `OffsetIndirect` case (rewriting the boolean-true form to the object
    form with the `encoding` annotation). The `LengthPrefixed` case is
@@ -217,19 +221,33 @@ validator for JSON-only schemas".
 
 ## Open Questions surfaced
 
-- **OQ-005** (new): Should `Union` materialization return a consistent
-  shape for byte-offset and field-name discriminators? The POC's byte-
-  offset path returns `{ "__discriminator": <u32>, ...variant-fields }`;
-  the field-name path returns the struct fields directly (the
-  discriminator is just another field). For `validate_bytes` callers,
-  the shape matters for how the validator dispatches. Resolve before
-  shipping the next POC round (SFTP Packet `validate_bytes`).
-- **OQ-006** (new): Should the builder spec's Example 3 (SFTP Packet)
-  wrap the `Union` in a `Struct`? Finding #1 above. Documentation fix,
-  not implementation.
-- **OQ-007** (new): `Bytes` materialization — lossy UTF-8 conversion
-  vs. a stricter byte-preserving validation form. Affects the SFTP use
-  case (binary `handle` and `data` fields).
+Three new open questions raised during the POC, now tracked in the
+central OQ tracker ([open-questions.md](../../../architecture/open-questions.md)):
+
+- **OQ-004** (open, low): `Discriminator::Field` name type — `&str` or
+  `String`? Raised in [builder.md](../../../architecture/builder.md)
+  §"Open Questions" during ADR-009 spec drafting. Doesn't block the
+  chunk header POC; must be resolved before the SFTP Packet POC's
+  field-name discriminator path. Full file:
+  [OQ-004](../../../architecture/questions/004-discriminator-field-name-type.md).
+- **OQ-005** (open, medium): `Union` materialization shape —
+  consistency between byte-offset and field-name discriminators.
+  The POC's `materialize_union_packed` returns inconsistent shapes
+  for the two kinds. Blocks the SFTP Packet `validate_bytes` POC (the
+  natural next round). Full file:
+  [OQ-005](../../../architecture/questions/005-union-materialization-shape.md).
+- **OQ-006** (open, low): Builder spec Example 3 — wrap the `Union`
+  in a `Struct`. The spec example shows a top-level `Schema::union_(...)`
+  which won't compile (`AlkTypeEngine::compile` requires
+  `AlkType:Struct` at the top level — Finding #1 above). Documentation
+  fix. Full file:
+  [OQ-006](../../../architecture/questions/006-builder-spec-example-3-wrap-union.md).
+- **OQ-007** (open, medium, one-way door): `Bytes` materialization —
+  lossy UTF-8 conversion. The current materializer uses
+  `String::from_utf8_lossy`, which corrupts non-UTF-8 bytes. Blocks
+  the SFTP use case for `validate_bytes` (binary `handle`/`data`
+  fields). Full file:
+  [OQ-007](../../../architecture/questions/007-bytes-materialization-lossy-utf8.md).
 
 ## Recommendation