Generate pybind declarations before bindings

[ProfFan]

Summary

• Split generated pybind code into declaration and binding phases.
• Register every class, enum, namespace, and container before binding any callable signatures.
• Recover registered Python objects during the binding phase to add constructors, methods, properties, and functions.
• Update the custom-template contract, documentation, golden outputs, and regression coverage.

Motivation

pybind11 constructs callable signatures when bindings are created. If a referenced wrapped type is declared by a later interface file, the signature can retain a raw C++ type name. Module-wide declare-then-bind ordering ensures all wrapped types are registered first.

Addresses borglab/gtsam#1824.

Validation

• Wrap CI sequence: 130 passed, 86% coverage.
• GTSAM stable and unstable Python extensions regenerated, compiled, and linked successfully using this branch.
• GTSAM stable Python suite: 510 passed, 6 skipped, 11 subtests passed.
• GTSAM current unstable Python suite: 6 passed, 6 subtests passed.

[dellaert]

Did you test it with GTSAM locally? I’d not merge until that works :-)

[ProfFan]

Yes