fix: implement BREAKX pure-Python backend (was stub of BREAK)

BREAKX previously inherited BREAK's γ() unchanged — yielding only once
and stopping on backtrack. Now correctly mirrors CSNOBOL4 L_BRKX/L_BRKXF:
steps one past each break character on backtrack, yielding at every
break-char position until the subject is exhausted.

Adds tests/test_breakx.py — 19 tests covering core semantics, BREAK vs
BREAKX contrast, edge cases, and integration.

Bumps version to 0.5.2.

version 0.5.1 — update project URLs to SNOBOL4-plus/SNOBOL4-python

version 0.5.0