Pin hatchling floor to 1.26

The build-system requires block had no minimum version on hatchling.
These pyprojects depend on force-include (hatchling 1.5+) and PEP 639
SPDX license handling (hatchling 1.26+), so a fresh build environment
with an older hatchling could silently produce a different wheel
layout or metadata. Pinning to >=1.26 uniformly across the four
packages documents the floor and keeps reproducibility consistent.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: antoineleclair

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["hatchling"]
+requires = ["hatchling>=1.26"]
 build-backend = "hatchling.build"
 
 [project]