breaking(c): get_callers/get_callees should take string signatures, not CFunction

[rahlk]

CAnalysis.get_callers() / get_callees() take a CFunction object; every other language facade takes string names/signatures (cldk/analysis/c/c_analysis.py:289,311).

Scope

• Accept string function names (plus optional file qualifier), matching the other facades.
• Consider fixing the odd capitalization of get_C_file / get_C_compilation_unit in the same pass.

Breaking change — v2 window.

Part of #193. Branch: major/issue-<n>.