v0.0.33

test: add regression test for v3-mirrored tree writing v4

TestRootIndexWritesV4WhenMirroredV3 directly captures the regression:
a HashTree with SchemaVersion=v3 (as mirrored from an old root) must
still emit a v4-format root docSchema from IndexReader(), and Rehash()
must produce SHA-256 of that v4 body.

Also expand the IndexReader() comment to clarify that
RMAPI_FORCE_SCHEMA_VERSION only overrides the serialized schema format,
not the hash algorithm — forcing v3 is for inspecting failure modes,
not a supported write path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>