fix: align Pattern pos/endpos signatures with re and add sliced-search fallback

[Qubitium]

Problem

re.Pattern methods accept pos/endpos positionally, but pypcre's Pattern.match/search/fullmatch/finditer/findall declared them keyword-only. Drop-in re-style code therefore failed:

pcre.compile("b").search(s, 2)
# TypeError: Pattern.search() takes 2 positional arguments but 3 were given

Fix

• Accept pos and endpos positionally on Pattern.match, search, fullmatch, finditer, and findall, matching the stdlib re.Pattern signature order; the pypcre-specific options argument remains keyword-only.
• For minimal non-C backends whose search() lacks pos/endpos support, Pattern.search() now falls back to searching a slice (subject[pos:endpos]) and returns an internal _OffsetMatch wrapper that shifts start()/end()/span()/regs back by pos, so reported spans stay absolute in the original subject while .string, .pos, .endpos, and .re still reference the original subject/pattern.

The C backend keeps forwarding pos natively: slicing alone would change semantics for anchors (^, $), \b, and lookbehinds relative to re, since they would evaluate against the slice boundary rather than the full string.

Tests

• New parity tests compare positional search/match/fullmatch/finditer/findall results (spans and pos) against stdlib re, including negative cases.
• New fallback test exercises a minimal backend without pos support and asserts absolute spans, regs, pos, string, and re on the returned match.

Full suite: 832 passed, 19 skipped (test_cache_strategy_benchmark failure is pre-existing on main and timing-related).