NilPy: len() on a str (only TPyList is accepted today)

Found 2026-07-19 while landing [[feature-nilpy-str-methods]]; pre-existing, NOT caused by that change.

s = "abc"
print(len(s))

pascal26:2: error: no overload of len matches these arguments ()

pylib declares only len(l: TPyList): Integer, so a string argument finds no overload. len() on strings is everywhere in Python generally and in uforth specifically, so milestone 1 needs it.

Fix shape: add len(const s: AnsiString): Integer to pylib next to the list one and let the existing overload machinery pick. Watch the landmine recorded in project_builtin_overload_shadows_used_unit — a builtin overload COMPETES with a used-unit routine and argument width steers the pick, so verify both len(list) and len(str) still resolve after adding it.

Note the error also dumps the compiler's raw MatchProcCall candidate table (arg[0] = ... / param[0] = ...) to stdout, which reads like stray debug output in a user-facing diagnostic. Possibly worth its own cleanup ticket.

Fixed 2026-07-19

function len(const s: AnsiString): Integer; overload; added beside the TPyList one. Both live in pylib, so this is plain same-unit overloading picked by argument type — the used-unit shadowing hazard in project_builtin_overload_shadows_used_unit does not apply. len(list) and len(str) both verified, including the while i < len(s) scanning idiom.

Landed with the join/split unit of [[feature-nilpy-str-methods]].