Add utility for structured updates in `ComponentArrays`

Motivation

In fitting or optimization workflows, it’s common to work with a structured ComponentArray template and update only a subset of its parameters during each iteration. While manual assignment is always possible using field names or indices, it’s often verbose and error-prone.

I'd like to have a utility function that enables updating parameters by name using existing axis metadata, without reconstructing the structure or altering types.

Proposed Functionality

Introduce update_component_array(default, update) that:

• Updates a ComponentArray default with values from another ComponentArray update
• Modifies only matching entries (partial updates supported)
• Preserves structure, type, and axis information of the original array

Example Usage

default = ComponentArray(sig = (mu = 1.0, sigma = 2.0), bg = 3.0)
update  = ComponentArray(sig = (mu = 1.1,), bg = 3.3)

result = update_component_array(default, update)

result.sig.mu    # 1.1
result.sig.sigma # 2.0
result.bg        # 3.3

Btw

Originally, I tried to implement deep_merge, but it's really hard to make it type stable. I'd be happy to have the update solution

Start by reviewing the existing ComponentArray axis metadata and named-component update APIs; the issue does not identify specific files or tests. Define update_component_array(default, update) so partial matching updates preserve the default's structure, type, and axes, then add coverage for the nested sig and bg example and type-stability behavior.