feat(pattern): add operator sugar for combinators

[sentomk]

Summary

Add operator forms for the pattern combinators: !p for neg(p), (a || b) for any(a, b), and (a && b) for all(a, b). The pattern algebra now reads like the boolean logic it models.

match(status) | on(
  !val<200>                  >> "error",
  (lit(301) || lit(302))     >> "redirect",
  (pred(is_json) && !lit(0)) >> "json payload",
  _                          >> "ok"
);

Changes

• negation.hpp: operator! equivalent to neg(p).
• combinator.hpp: operator|| equivalent to any(a, b); operator&& equivalent to all(a, b).
• The overloads are declared in ptn::pat::base: every pattern derives from pattern_base, which makes that namespace ADL-associated for all pattern types. (Concrete patterns live in ptn::pat::detail, and ADL does not ascend enclosing namespaces — declaring the operators in ptn::pat would make them unfindable.)
• Operands are constrained to patterns that are not guard predicates, so guard-level && / || inside [...] keep their pred_and / pred_or semantics — verified by test.
• Docs: docs/api.md combinator sections, roadmap WIP entry, README negation example.

Testing

• Nine new cases in tests_combinator.cpp: type identity of !p vs neg(p), or/and/chained-or/double-bang behavior, sugar mixing pattern kinds (is<int> || is<std::string>), and guard-operator non-interference.
• Full suite: 204/204 passing locally (Clang 20, C++17).
• One caveat, documented: >> binds tighter than ||/&&, so combined patterns need parentheses in a case — (lit(1) || lit(2)) >> handler. Unary ! needs none.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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