1.3.0-beta3: operator-experience fixes from the rc2 production-soak

Stepping back from rc to beta after the 1.3.0-rc2 install reached
the actual PURGE code path for the first time on a Cloudflare-
fronted Jelastic-managed LSWS Enterprise environment and surfaced
operator-experience gaps that warranted a fresh soak window before
re-claiming rc quality.

What landed since 1.3.0-rc2 (commit f037d9b):

- 442e93f: actionable error messages. Suggested URL in the no-host
  failure reason changed from https://your-site.example.com to
  http://localhost/ (the previous example sent operators behind any
  CDN into a 400 trap). HTTP non-2xx purge failures now surface
  operator-actionable reasons via the new formatNon2xxReason()
  helper, detecting Cloudflare's HTTP 400 + CF-Ray signature
  specifically and pointing operators at origin-direct purge_host
  alternatives. Same change applied to the settings form
  description and placeholder.

- 88d4398: purger logging now routes through @logger.channel.
  lscache_purger via constructor injection. The channel was
  registered in services.yml since 1.0.0 but never used; the plugin
  defaulted to PurgerBase's framework channel. drush watchdog:show
  --type=lscache_purger now returns purge-warning entries instead
  of "Unrecognized message type". Messenger-surfaced failure
  reasons now apply to every failure path (no-host, HTTP non-2xx,
  transport error), not just the no-host case rc2 covered.

- 89a1150: two new doc sections in docs/htaccess-gotchas.md.
  "LSWS-side PURGE handling" makes explicit that HTTP 200 from
  LSWS means "request accepted at the protocol level," not "entry
  evicted." On managed-LSWS environments (Jelastic, Cloudways)
  and on Enterprise installs with admin-port routing, the two can
  be distinct states. Documents the diagnostic, LSWS configuration
  requirements, and operator workarounds. "LSCache vs Drupal's
  dynamic_page_cache UNCACHEABLE hint" covers the case where the
  two cacheability signals disagree, documents how to opt out via
  response policy when needed.

- 5593f00: ROADMAP records the rc2 -> beta3 step-back with the
  rationale and the deferred-to-1.3.1/1.4.x list (post-purge
  sentinel verification flag, everything-invalidation type, the
  operator-experience polish items).

Pure operator-experience + docs scope. No API contract changes;
the "HTTP 200 = SUCCEEDED" mapping is unchanged. Finding 7 (auth-
served-as-anon variant) was canary-clean on rc2 across 81
controlled requests; treating that finding as resolved pending the
invalidation-driven re-test once LSWS-side PURGE works in the test
environment.

Soak target ~5-7 days. If clean, promotes straight to rc3, then
~2-week soak, then 1.3.0 stable. If anything new surfaces, beta4
cycle.