fix(blocks): attribute values are validated before any write; null/bool/objects refused by tag

block_insert and block_set_attributes applied str() to whatever arrived, so
a JSON null became the text None and a non-mapping failed after the INSERT
was written. validate_attribute_values (backends/block_specs.py) runs first
on both engines: strings pass the one-line rule, ints and finite floats are
written as plain digits, everything else is a TypeError naming the tag.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

server.py

@@ -3011,7 +3011,15 @@ async def block_insert(
     layer: Annotated[str | None, "Layer name"] = None,
     ctx: Context = None,
 ) -> dict:
-    """Insert a block and optionally set attribute values."""
+    """Insert a block and optionally set attribute values.
+
+    Refused before any write, on both engines: an undefined or layout block
+    name, and an attribute value that is not text — a string is written as
+    is (one line), an int or finite float as its plain digits (`101` →
+    `"101"`), while `null`, booleans, lists and objects are refused naming
+    the tag (they used to be written as their Python repr). Values for tags
+    the block does not define are ignored.
+    """
     await ctx.info(f"Inserting block '{name}' at ({x},{y})")
     result = await _backend(ctx).block_insert(
         name, x, y, scale_x, scale_y, rotation, attributes, layer
@@ -3085,6 +3093,11 @@ async def block_set_attributes(
 ) -> dict:
     """Update attribute values in a block reference.
 
+    Same value rule as `block_insert`: strings and numbers are written,
+    `null` / booleans / lists / objects are refused naming the tag before
+    anything changes. `updated_tags` lists the tags that exist on the
+    reference and were written; unknown tags are ignored.
+
     A multi-line attribute takes `Line1\\PLine2` and keeps every line; the
     value written is the one `block_get_attributes` reads back and
     `block_explode` bursts, on both engines.