Execute an interceptor chain with enter/execute/leave phases.

Args:
  ctx          - the initial context map.
  interceptors - sequential collection of interceptor maps.
  execute-fn   - 1-arity fn [ctx] -> task; performs the core operation.
  opts         - (optional) map; :max-interceptor-depth overrides the default 64.

Returns a Missionary task resolving to the final context map on success,
or rejecting with the original throwable on failure.

Pipeline phases:
  1. Enter:   each interceptor's :enter handler runs in queue order (FIFO).
  2. Execute: execute-fn is called with the context after all :enter handlers.
  3. Leave:   each interceptor's :leave handler runs in reverse order (LIFO).

On failure in any phase, run-error-stage is invoked with the appropriate
stack of interceptors for cleanup. The original throwable is always re-thrown
after error handlers complete.

Each stage is isolated in a nested m/sp:
  JVM restriction: recur cannot appear in tail position inside try.
  Cancellation:    each m/? is a Missionary cancellation checkpoint.

Throws (synchronously):
  ex-info :clj-r2dbc/limit-exceeded when interceptor chain exceeds
  :max-interceptor-depth (default 64).

missionary.Cancelled is never swallowed - it propagates through error handlers
and triggers physical Subscription.cancel() on R2DBC publishers.