Composite attribute support

[colemancda]

Adds composite attribute support, following CoreModel #11. Requires CoreModel 2.11.0.

Storage

A composite maps onto MongoDB's embedded document — its native shape for structured values, so no flattening or serialization is needed:

{ "location": { "latitude": 40.7, "longitude": -74.0 } }

Element key paths are the dotted paths the server already understands, so location.latitude > 30 becomes an ordinary query with no rewriting:

{ "location.latitude": { "$gt": 30 } }

Decoding iterates the declared elements rather than the stored document, so a stale field the schema no longer declares is ignored rather than mis-typed, and an element missing from the document decodes as .null so the decoded shape always matches the schema.

The in-memory evaluation fallback (used for predicates the server can't run, e.g. custom functions) also descends into composite elements.

Also included

.arithmetic expressions (new in CoreModel 2.11.0) are rejected for server-side queries and return nil from the in-memory fallback rather than being silently mis-evaluated; both marked TODO. containsFunction does recurse into arithmetic operands, since an operand can contain a function call.

Testing

12 tests covering BSON encode/decode, nested composites, missing and unknown elements, wrong element types, null handling, key-path resolution (including the empty-key-path edge case), and the generated server queries for flat and nested element paths. All pass.

Caveat: these are server-free unit tests. The composite logic lives in the conversion and key-path code they cover, but the end-to-end query path has not been exercised against a live mongod — no local instance was available. Worth an integration run before merging.