Fix C++ template call segfault

[Smiie-2]

This fixes a crash in the C/C++ LSP path when resolving nested template function calls with default template arguments. The underlying issue was that unbound template parameters could flow through substitution as
NULL, and call-expression return handling assumed the substituted type was always valid.

Example
MangoHud’s src/hud_elements.cpp was one reproducer, but the fix targets the general case of calls like a nested template function inside another call expression

What changed:

• Preserved unbound template parameters during type substitution instead of turning them into NULL.
• Added a guard before unwrapping substituted call return types.
• Improved template-function lookup in the C++ call resolver path.
• Tightened pending template-call deduction so it respects the registered formal parameter count.
• Made C++ template-declaration unwrapping handle nested template declarations more reliably.
• Added regressions for the general substitution bug and for the nested template-call crash pattern.

Verification:

• make -f Makefile.cbm cbm
• make -f Makefile.cbm build/c/test-runner
• clsp_nocrash_template_function_multi_param_nested_call PASS
• typerep_substitute_unbound_param_preserved PASS