str.rsplit()/partition()/rpartition() missing

Found by proactive CPython-diff sweeping. split() existed with its 0/1/2-argument forms, but its sibling rsplit() did not, nor did partition()/rpartition() — all real, commonly-used str methods ("unsupported str method" error listing every OTHER str method NilPy has).

Fix

• compiler/builtin/pylib.pas: pystr_rsplit_sep_max (right-anchored maxsplit — only the 2-argument form of rsplit needs its own algorithm; rsplit()/rsplit(sep) give the identical result to split()/split(sep) since only a maxsplit limit depends on which end it's anchored to). pystr_partition/pystr_rpartition — a 3-tuple (before, sep, after) at the first/last occurrence, or (s,'','') / ('','',s) when the separator is absent, matching CPython exactly.
• compiler/pyparser.inc: wired rsplit into PyStrMethodInfo reusing split's dispatch shape (a new wantArgs = -8 case in PyParseStrMethod that reuses pystr_split_ws/pystr_split_sep for 0/1 args and only calls the new right-anchored function for the 2-arg form); partition/ rpartition as ordinary 1-argument str methods. Updated the "unsupported str method" error's method list to include all three.

Verified against CPython (empty-string edge case, separator absent, maxsplit values). Regression test test/test_nilpy_str_rsplit_partition.npy (gated in test-nilpy). Self-host confirmed byte-identical via make pxx-debug.

Log

• 2026-07-31 — resolved, commit HEAD.