refactor: git mv validate.py and validate_types.py into dandi/validate/ subpackage

Pure file move with no content changes, plus __init__.py re-exports for
backward compatibility. Imports will be updated in the next commit.

Co-Authored-By: Claude Code 2.1.63 / Claude Opus 4.6 <noreply@anthropic.com>

Author: yarikoptic

dandi/validate/__init__.py

@@ -0,0 +1,46 @@
+"""Validation of DANDI datasets against schemas and standards.
+
+This subpackage provides validation functionality for dandisets, including:
+- DANDI schema validation
+- BIDS standard validation
+- File layout and organization validation
+- Metadata completeness checking
+
+Submodules:
+- core: Main validation functions (validate, validate_bids)
+- types: Data types and models (ValidationResult, Origin, Severity, etc.)
+- io: JSONL read/write utilities for validation results
+"""
+
+from .core import validate, validate_bids
+from .types import (
+    ORIGIN_INTERNAL_DANDI,
+    ORIGIN_VALIDATION_DANDI,
+    ORIGIN_VALIDATION_DANDI_LAYOUT,
+    ORIGIN_VALIDATION_DANDI_ZARR,
+    Origin,
+    OriginType,
+    Scope,
+    Severity,
+    Severity_,
+    Standard,
+    ValidationResult,
+    Validator,
+)
+
+__all__ = [
+    "ORIGIN_INTERNAL_DANDI",
+    "ORIGIN_VALIDATION_DANDI",
+    "ORIGIN_VALIDATION_DANDI_LAYOUT",
+    "ORIGIN_VALIDATION_DANDI_ZARR",
+    "Origin",
+    "OriginType",
+    "Scope",
+    "Severity",
+    "Severity_",
+    "Standard",
+    "ValidationResult",
+    "Validator",
+    "validate",
+    "validate_bids",
+]