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/architecture/builder.md

@@ -609,9 +609,9 @@ let op_errors = vec![
 - **OQ-004** (open): `Discriminator::Field` name type — `&str` or
   `String`? The builder consumes `Self` on each setter, so the
   discriminator's `name` needs to outlive the `Schema` it's embedded
-  in. `&str` borrows; `String` owns. Resolve during implementation
-  (likely `String` for ownership simplicity, since the builder owns
-  its content until `.build()`).
+  in. `&str` borrows; `String` owns. Tracked in
+  [open-questions.md](open-questions.md); full file:
+  [OQ-004](questions/004-discriminator-field-name-type.md).
 
 ## References