grpc 1.0 migration + Apache-2.0 license fix (v0.5.0)

[nyo16]

Summary

Migrates the pool to grpc ~> 1.0 (from 0.11.5) and reworks active
disconnect detection for grpc 1.0's new client-only connection model. Also
aligns the license declaration to Apache-2.0 (#6). Version bump to 0.4.0.

Why disconnect detection had to change

grpc 1.0's Gun adapter owns the socket inside its own supervised
ConnectionProcess. Empirically (spike):

• On a drop, only the inner gun process dies — the ConnectionProcess
  (conn_pid in channel.adapter_payload) lingers as a zombie. So the
  plan's original idea of Process.monitor(conn_pid) can't detect drops.
• grpc emits no connection-level telemetry (RPC spans only), so attaching
  to grpc telemetry doesn't work either.
• The old :gun_down/:gun_error handlers in Worker were dead code — those
  messages now go to the adapter's process, never to us.

Fix: the worker monitors the inner gun process (read via
:sys.get_state(conn_pid).gun_pid, guarded, falling back to conn_pid) paired
with adapter_opts: [retry: 0], so gun fails fast on a drop and the pool's own
Backoff governs reconnection. GRPC.Stub.disconnect/1 reaps the zombie
ConnectionProcess on teardown.

This also restores fast-fail connects: a dead endpoint now errors in ~0ms
instead of stalling ~5s/attempt. The full test suite drops from ~364s to ~2.5s.

⚠️ Breaking change (telemetry)

The [:grpc_connection_pool, :channel, :gun_down] and :gun_error events are
removed (unreachable under grpc 1.0). A single adapter-agnostic
[:grpc_connection_pool, :channel, :connection_down] event (metadata
pool_name, reason) is emitted instead. :disconnected and
:reconnect_scheduled are unchanged. Consumers should switch to
:connection_down (or :disconnected).

Changes

• deps: grpc ~> 1.0, gun ~> 2.2 (now a direct dep — grpc 1.0 makes it
  optional), cowboy ~> 2.14 (test); :public_key/:ssl in extra_applications.
• worker: monitor the gun process, demonitor(ref, [:flush]) on disconnect,
  GRPC.Stub.disconnect/1 teardown; removed dead gun/mint message handlers.
• config: adapter_opts: [retry: 0] (fast-fail + pool-governed reconnect).
• tests: drive real drops via a Cowboy h2c server (test/support/)
  instead of synthetic gun messages; deleted obsolete disconnect_fix_test.exs;
  fixed a latent flaky test that hardcoded the live emulator port 8085.
• license: mix.exs + README now declare Apache-2.0 to match the
  committed LICENSE file (fixes the Hex.pm mismatch).
• docs: CHANGELOG 0.4.0, README install snippet + telemetry section.

Review notes

The one thing worth a close look: the worker reads gun_pid via
:sys.get_state/2 on grpc's adapter process — a deliberate, guarded coupling to
a grpc internal. It's the only way to detect drops under grpc 1.0 given the
zombie ConnectionProcess and the absence of connection telemetry; it falls
back to monitoring conn_pid if the internal state shape ever changes.

Verification

mix format --check-formatted ✓ · mix compile --warnings-as-errors ✓ ·
mix credo (no issues) ✓ · mix dialyzer (0 errors) ✓ ·
mix test → 75 passed, 3 excluded (emulator), ~2.5s ✓

Closes #6