A struct-field function-pointer array is not callable

Found and fixed 2026-08-20 in the same sitting, and by the same gcc differential probe, as its sibling. The measurement table, both root causes, the test and the gate are written up once, in:

done/bug-c-a-local-typedef-d-function-pointer-array-is-not-callable.md

This ticket exists so that slug resolves — the code comment in compiler/cparser.inc (the new AN_INDEX over AN_FIELD arm of CalleeSig) and test/cfnptr_array_callable.c both name it.

In one line: CalleeSig gained an arm for an AN_INDEX whose left is an AN_FIELD, reading the field's per-element signature from RecFieldElemProcSig — the same channel (*s.pf)(args) already used — with a fallback to RecFieldProcSig.