feat(dllm): backend packaging, gallery index, CI matrix

Registers the dllm backend across every surface: backend gallery index
(cpu amd64+arm64 with manifest merge, cuda13, l4t-cuda13 for GB10-class
hardware; no darwin per engine scope), top-level Makefile targets,
bump_deps pin tracking for DLLM_VERSION, and the curated known-backends
list for /backends/known (pref-only: auto-detecting on .gguf would
shadow llama-cpp). Note: image builds and the nightly bump leg stay red
until github.com/mudler/dllm.cpp is published (planned at merge time).

Assisted-by: Claude Code (Fable 5)
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
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