Support numeric constraints for decimal.Decimal

[Siyet]

Reopening #998 (auto-closed when fork was deleted).

Fixes #683.

Previously, the numeric constraints gt, ge, lt, le, and multiple_of in msgspec.Meta were only valid for int and float types. This PR extends support to decimal.Decimal.

PR #692 by @cocorigon has been open since May 2024. The author has since explicitly declined to continue it and invited others to take it over. This is a fresh implementation with documentation and full test coverage.

Changes

• _core.c: Added five new TypeNode constraint flags backed by PyObject* slots storing Decimal instances. A new ms_check_decimal_constraints() function is called after every Decimal decode path.
• __init__.pyi: Updated Meta.__init__ parameter types to include Decimal.
• docs/constraints.rst: Updated Numeric Constraints section to mention decimal.Decimal.
• tests/unit/test_constraints.py: Added TestDecimalConstraints covering all constraint types.

Notable design decisions

• multiple_of for Decimal uses Python's % operator, which gives exact arithmetic.
• Constraint bounds passed as float are converted to Decimal via string representation to preserve precision.

[ofek]

It looks like the Git history needs to be fixed on this one.

[Siyet]

The failing build job here is unrelated to this PR: it's the link checker tripping on the Pydantic docs redirect (docs.pydantic.dev/latest/ → pydantic.dev/docs/validation/...), which is fixed in #1008. Once #1008 lands and this branch is rebased, CI should go green.

[Siyet]

@ofek done — force-pushed a clean rewrite of the branch:

• Squashed into a single commit with a proper message and Closes #683 reference.
• Rebased onto current main, dropping all the leftover commits from the deleted fork (CODEOWNERS, package rename, doc-link rewrites, plus changes that belong to other PRs).
• Scope is now strictly the four Decimal-relevant files: _core.c, __init__.pyi, docs/constraints.rst, tests/unit/test_constraints.py. Diff stat is +353/-52.
• MS_CONSTR_DECIMAL_* flags claim bits 36-40, the lowest unused slots in TypeNode->types. If Support Literal[True] and Literal[False] types #1004 lands first, I'll rebase to whatever bits are next free.

Build job is still red on the link checker but that's the unrelated #1008 fix; nothing in this branch touches it.