Worker init parity — st_init_multi + COLI_MODEL_DIRS

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What to build

Make the worker's init match a sharded coordinator: honor COLI_MODEL_DIRS via st_init_multi instead of the single-directory st_init, so a worker can resolve expert tensors split across drives. Surface a clear worker error (not a mid-connection fatal) when an expert tensor can't be resolved. Do not port the rest of model_init (tokenizer/KVCache/router/embed/MTP) into the worker — dead weight for an expert-only worker.

Acceptance criteria

• Worker resolves expert tensors through st_init_multi honoring COLI_MODEL_DIRS.
• An unresolvable expert tensor surfaces as a clear worker-side error, not a mid-connection fatal/exit.
• Single-directory models still load identically (no regression).

Environment: codegraph-indexed — run codegraph explore first; refs in /home/eli/dev/_reference-docs-and-repos/colibri.