Adds the full README (quickstart, interaction styles, privileged intents,
cache, REST, slash registration, event semantics, Nostrum porting guide,
reliability design), an examples/echo_bot.exs Mix.install script, ex_doc
config in mix.exs, and rewrites Dexcord.Gateway's moduledoc to describe
the finished reliability behavior instead of stale Phase 0-1 scope notes.
- fix heartbeat timer loss in :hello_wait (re-arm during identify-gap wait)
- honor server op 1 in :identifying/:hello_wait, not just :connected/:resuming
- full op 9 handling: d:false in :resuming re-IDENTIFYs on the same socket after
the mandated 1-5s; d:false elsewhere floors the reconnect; d:true keeps session
- resume-loop cap (5 consecutive failed resumes -> fresh IDENTIFY) via Session ETS
- explicit conn_gen/stale-socket guard backstopping Mint's :unknown
- bounded TCP connect + configurable handshake watchdogs so a stalled reconnect
self-heals instead of wedging
- FakeGateway scripted Bandit/WebSock test server + TestHandler
- integration suite (happy/zombie/kill/close-codes/op7/op9/replay/cap/backoff/gap/
stale), fatal-path end-to-end, and unit tests for resume counter + backoff math
