Need a more powerful and clean signature merging functionality

[thorwhalen]

Sig.merge_with_sig is horrendously designed and not even that expressive.

We need to think more carefully of its design and interaction with any signature-merging functionalities (for example in meshed).

There are some other signature merging functions laying around. Notably, meshed.util.conservative_parameter_merge which is fairly clean, but only takes care of kind, default and annotation conflicts.

[thorwhalen]

Possible duplicate issue: Write a general parameters merging function.

[thorwhalen]

Still valid: signatures.py is now 5564 lines and Sig.merge_with_sig (line 2111) remains the primary merge entrypoint (Sig.__add__ at line 2257 delegates to it), with ch_to_all_pk still a separate conflict-resolution knob layered on top. No redesign has landed since this was filed. This is the same underlying ask as #5 (2021, "write a general parameters merging function") -- #43 is the more concrete complaint about the current merge_with_sig specifically. Also related: #72 and #49 both hinge on what "compatible signatures" means, which any merge redesign would need to define too. Not attempting a redesign here -- this is a genuine architecture decision (single deliberate design, not several independent small fixes) that needs Thor's input on the target API shape; labeling manual-task accordingly.