Fix split GGUF stage inventory discovery

[i386]

Summary

Fixes direct split-GGUF stage inventory discovery for models whose first shard is metadata-only, stacked on #1194.

Root cause

Stage inventory discovery inferred the layer count only by scanning tensor names. DeepSeek-V4 split GGUFs can place metadata in the first shard while storing all tensors in later shards, so the tensor scan returned no layers and the peer advertised an empty inventory. Split readiness then waited indefinitely even though the mesh connection itself was healthy.

Fix

• Reuse direct-GGUF shard expansion to validate and enumerate every shard.
• Read the layer count from compact GGUF metadata first, retaining tensor inspection as a fallback.
• Compute the advertised source size across all validated shards.
• Add a regression test for a metadata-only first shard.

Validation

• just build
• cargo test -p mesh-llm-host-runtime --lib inventory_source_resolves_metadata_only_first_shard
• cargo fmt --all --check
• cargo check -p mesh-llm-host-runtime
• cargo check -p mesh-llm
• cargo clippy -p mesh-llm-host-runtime --all-targets -- -D warnings -A unfulfilled-lint-expectations
• cargo clippy -p mesh-llm --all-targets -- -D warnings -A unfulfilled-lint-expectations

Strict clippy remains blocked by 28 pre-existing unfulfilled_lint_expectations errors in the parent PR branch; none are in this diff.

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