feat(cypher): multi-argument scalar functions (suite tier 2b)

Add an args-list (cbm_func_arg_t) to return items and parse comma-separated
arguments (var.prop or string/number literals), enabling multi-argument scalar
functions in projections:

- coalesce(a, b, ...)        first non-empty value
- substring(s, start[, len]) 0-indexed substring
- replace(s, from, to)       replace all occurrences
- left(s, n) / right(s, n)   leading/trailing n chars

Arguments are freed in free_return_clause (ASan-clean). Unit test
cypher_func_multiarg plus smoke coverage for substring + coalesce.

Refs: Cypher read suite (tier 2b)