docs(readme): link ecosystem badges + move Beta callout to bottom

Three operator-feedback corrections to the README:

1. Every ecosystem badge in the top row now links to a real
   destination — Home Assistant -> integrations/home-assistant.md,
   Matter -> ADR-122, Apple Home -> user-guide-apple-homepod.md,
   Google Home + Alexa -> the HA integration doc (both ecosystems
   reach RuView through HA's bridge today). Added an Alexa badge
   alongside the existing four so all four major ecosystems are
   represented. Dropped the now-redundant separate "HomePod
   Integration" badge — the Apple Home badge linking to the same
   guide is enough.

2. Beta callout moved from line 14 (under the hero image) to a
   dedicated `## Beta software` section immediately before the
   License. The callout's content is unchanged; it just no longer
   gates the elevator pitch. Readers see the value proposition
   first, the caveats at the bottom alongside license + support.

3. The intro paragraph ("Turn ordinary WiFi into ...") now ends
   with a one-line summary of native ecosystem support naming all
   four — Home Assistant, Apple Home & HomePod, Google Home, Alexa —
   plus the Matter endpoint, each linked. The previous mention of
   ecosystems was buried further down the page; this surfaces it
   in the intro where the user reads first.

Co-Authored-By: claude-flow <ruv@ruv.net>