NilPy lexer: hex/octal/binary int literals, triple-quoted strings, string line counting

Part of [[feature-nilpy-corpus-uforth]] milestone 1 (first parse blockers in uforth.py, file order).

Bugs/gaps

1. Hex literals silently misparse: SYS_DATA_START = 0x10000 lexes as integer 0 followed by identifier x10000 — no error, wrong value. uforth.py's memory map (lines 40-55) is all hex. Add 0x/0X, 0o/0O, 0b/0B prefixes + _ digit separators (also in decimal/float).
2. Triple-quoted strings: """docstring""" currently lexes as empty string + runaway single-quote string swallowing the docstring body. Add real '''/""" handling (multi-line, no escape of the quote run).
3. SrcLine not incremented inside string literals — every newline consumed inside a string body drifts all subsequent error line numbers (observed: uforth.py line 61 reported as 50).

Gate

test-nilpy green (+ new regression test_nilpy_literals.npy), self-host byte-identical, testmgr quick.