0.19.0-alpha

Added

• A third, optional layer axis: --host/-H (docs/HOST_LAYER_DESIGN.md) — raised by the
  repo owner from a real deployment shape: a load-balanced Production environment where
  individual servers need genuinely different config from each other, not just from other
  clients/environments. Rejects the hyphenated Client-Host naming workaround (e.g.
  Acme-192.168.10.10) in favor of one more optional extends hop,
  .configtransform/Clients/<C>/<E>/Hosts/<H>/configtransform.json, reusing the
  self-describing-layers chaining as-is. LayerPathResolver.Resolve gains an optional host
  parameter (requires client+environment, mirroring the existing client-requires-
  environment rule one level up); applies uniformly to a plain resolve, --dry-run/--diff,
  --list (both modes), and set. set into a new Host layer defaults its extends to the
  matching Client/Environment layer, the same convention every other layer's default already
  follows, one level deeper (SetTargetResolver). init gains a repeatable --host flag
  (quiet mode) and one more interactive prompt (asked only once a client was given), cross-
  multiplying with every declared client × environment pair the same way clients already
  cross-multiply with environments (InitPlanner), plus a matching case-insensitive-collision
  check. -H (capital) is the short alias — -h stays --help, a deliberate, documented
  tradeoff (docs/HOST_LAYER_DESIGN.md's decision log #2). Verified to need zero changes, by
  reading the actual code rather than assuming: LayerChain's extends-chain walk and
  --list --resource's reverse lookup already treat directory depth as arbitrary, and every
  format engine's Merge method only ever sees an ordered patch-path list, never a layer's
  position in the tree. 31 new tests across ConfigTransform.Core.Tests/
  ConfigTransform.Cli.Tests. init --template's own --host-aware variant
  (docs/HOST_LAYER_DESIGN.md decision log #7) is deliberately deferred to a separate PR — the
  fixed canned tree stays the minimal, no-decisions starter it's always been.