v0.7.2 — optional per-request scope

Feature release: a new opt-in knob to drop the per-request async scope on hot paths.

Added

• HttpServerConfig::setRequestScope(bool) / isRequestScope() — opt out of the
  per-request child async scope (default on, behaviour unchanged). When off, each
  H1/H2/H3 handler coroutine reuses the connection scope directly instead of minting a
  fresh per-request child, saving two allocations (emalloc/efree) per request. The
  setter is chainable and locks once the config is handed to a server.

  Disabling it means Async\request_context() resolves to null for that request
  (use the ?-> operator) — there is no per-request context subtree. Only disable it
  for handlers that never rely on per-request context (e.g. throughput benchmarks). The
  knob propagates correctly across setWorkers(N > 1) via the cross-thread shared-config
  snapshot.

Tests

• server/core/049-request-scope-setter — default / toggle / chainable / locked-guard,
  plus scope-OFF serving with Async\request_context() asserted null.
• server/core/050-request-scope-workers — the knob is honoured on worker threads
  (setWorkers(2)), guarding the shared-config propagation path.

Also folds in 0.7.1 (HTTP/3 bidi stream-credit fix, #79), which shipped tagged but
without a changelog entry.