Reframe protocol as REST peers; mDNS-SD discovery; explicit no-ack

Three structural shifts:

1. Discovery section spells out mDNS-SD. The ESP32 runs the ESP-IDF
   mdns responder and serves all .local records itself — A, SRV, PTR,
   TXT. Scrypted browses _scrypted-viewport._tcp.local with a Node
   mDNS-SD library that hits the multicast layer directly. Scrypted
   uses the IP from the SRV/A record, NOT OS-level .local hostname
   resolution. Re-browse every few minutes for DHCP renumbering.
   Manual host:port fallback for non-mDNS deployments.

2. Drop "callback" framing. The device and Scrypted are HTTP peers,
   both exposing POST /state with the same body shape {viewport,state}.
   Either side can push to the other to set state. Section is renamed
   "Device -> Scrypted POST /state". No new vocabulary or semantics —
   just the truth about what the protocol is.

   - state_post_failures (was: callback_failures)
   - state_client.c (was: callback_client.c)
   - "callback" replaced with "outbound POST" / "inbound POST" / just
     "POST /state" throughout.

3. Explicit no-application-level-ack clause. HTTP 2xx is transport-
   only. Device does not retry, does not block subsequent state
   changes on the response, does not treat 5xx as anything beyond a
   counter increment. Idempotency + /frame 409 + each side's idle
   timer cover every failure mode. Scrypted-side logic must not wait
   for the device to confirm a state change.

Also: Scrypted Integration code in v2 guide rewritten to discover via
bonjour-service browse rather than hardcoded viewport-<name>.local
URLs. urlFor(name) function resolves the discovered IP per call.
mdns_service.c module spec now explicitly notes Scrypted uses the
browse, not the hostname.

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