Create a cache for loaded calendars

[wilsonfreitas]

Problem

Calendar.load(...) rebuilds calendars on every call, even when the same named source is loaded repeatedly. That now affects packaged calendars as well as the optional provider integrations (PMC/, XCAL/, WORK/).

Refined scope

1. Add an internal cache for repeated Calendar.load(name=...) calls.
2. Cover packaged calendars and the existing named providers:
   • ANBIMA, B3, Actual
   • PMC/<calendar>
   • XCAL/<calendar>
   • WORK/<code>
3. Keep filename=... loads out of the initial cache scope to avoid stale-file invalidation complexity.
4. Preserve behavior safety: the cache must not leak shared mutable Calendar instances across callers.
5. Keep the NumPy-backed and legacy loaders aligned.

Acceptance notes

• Repeated named loads should avoid reparsing or rebuilding from the source.
• A caller mutating a returned calendar object must not affect later loads.
• Tests will likely need an internal cache reset helper, but that does not need to be part of the public API.
• list_calendars() is discovery only; this issue is about load-time reuse.