Correct OTLP metrics/logs behaviour to 404, and start Phase 2

Alloy only registers an OTLP/HTTP route for a signal with an output, so
POST /v1/metrics and /v1/logs return 404 rather than being accepted and
silently dropped (verified live against otlp.vione.cloud). Fix the claim
everywhere it appeared. Also mark Phase 2 in progress.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -263,7 +263,8 @@ Three constraints inherited from that platform, worth knowing before you debug
 anything:
 
 - **Alloy's OTLP receiver forwards traces only.** OTLP metrics and logs are
-  accepted and silently dropped — by design; the platform's diagram labels that
+  rejected with a 404 on `/v1/metrics` and `/v1/logs`, and the only sign is an
+  export error in the sending app's logs. This is by design; the platform's diagram labels that
   input "spans + resource attrs". Metrics reach Prometheus by **direct scrape** of
   each service's `/metrics` endpoint, registered in `prometheus_extra_scrape_jobs`.
 - **Log→trace correlation matches `trace[_-]?id["=: ]+([a-fA-F0-9]+)`,