Improve Meta overloads - #700

The overloads look right. I checked them against the runtime constraint validation across the full combination matrix (single constraints, every gt/ge x lt/le pairing, numeric mixed with pattern/min_length/max_length/tz, and metadata-only) and the type-checker verdict matches Meta.__init__'s runtime ValueErrors exactly, with no false positives or negatives: every combo the runtime rejects is a call-overload error, and every runtime-valid combo type-checks. mypy and pyrefly are both clean on basic_typing_examples.py; pyright is covered by CI plus the inline ignore markers. Nice that this lifts the runtime checks to type-check time.

One coordination note: #1116 (open) also edits the from typing_extensions import ... line (it adds disjoint_base), so whichever of these two lands second will need a trivial one-line rebase on that import.

Approving. One scope question, since the title mentions PEP 746: this diff is the type-safe Meta constructor overloads only, not the Annotated-metadata (__supertype__) side of PEP 746. Is that part still planned here, or is this PR now just the constructor overloads?