Adopt a centralized constraints system at the monorepo root.

### Summary of the feature request

Consolidate package level constraints to a single source at the monorepo root.

**Current State**: 27 packages define this file.
**Similarity Profile**: 61.0% similarity. Line counts vary between 0 (empty files acting as no-ops) and 124 lines.
**Why it's a prime candidate**: Pinned test dependencies are constantly updated. Doing this across 27 packages creates highly fragmented and asynchronous dependency constraints.

**Recommendation**:
Adopt a single centralized constraints system. A centralized constraints directory at the root (e.g. `testing/constraints/constraints-3.10.txt`) can be read by Nox across all packages.
Alternatively, use dynamic resolution or share a common base constraints file that packages can pull via a local script.