fix: full depth tracking across Codable container hierarchy

[hamchapman]

This change includes 2 main changes.

1. Enforcing maximumDepth in CodableCBORDecoder

Previously, the maximumDepth option was defined but not actually used in
CodableCBORDecoder, creating a DoS vulnerability where deeply nested
structures could cause stack overflow.

Problem:

1. CodableCBORDecoder had a maximumDepth property but it wasn't included
   in the options computed property, so it defaulted to .max
2. SingleValueDecodingContainer called CBOR.decode() without passing options
3. KeyedDecodingContainer and UnkeyedDecodingContainer created CBORDecoder
   instances without passing options
4. This meant depth checking was never enforced for Codable decoding

Fix:

• Add maximumDepth property to CodableCBORDecoder (public API)
• Include maximumDepth in options computed property
• Update setOptions() to set maximumDepth
• Pass options to all CBOR.decode() calls in SingleValueDecodingContainer
• Pass options to CBORDecoder() calls in KeyedDecodingContainer
• Pass options to CBORDecoder() calls in UnkeyedDecodingContainer

2. Implement full depth tracking across Codable container hierarchy

Changes:

• Added currentDepth field to _CBORDecoder and all container classes
  (KeyedContainer, UnkeyedContainer, SingleValueContainer)
• Thread currentDepth through all container creation and nested decoding
• Check depth before creating keyed/unkeyed containers in _CBORDecoder
• Increment depth when creating nested decoders in decode() methods
• Pass incremented depth through superDecoder() methods

Previously, each CBOR.decode() call would reset currentDepth to 0, allowing
deeply nested structures to bypass the depth limit. For example, a 10-level
nested array with maximumDepth=5 would incorrectly succeed. Now depth is
properly tracked across the entire container hierarchy.