Add standalone HF v2 shard emitter

[chboishabba]

This adds a minimal standalone producer for HF-backed partial loading.

What it adds:

• tools/emit_zelph_hf_v2.py
  • stdlib-only
  • reads a Zelph .index-file sidecar
  • emits one shard file per section-local chunk
  • writes a zelph-hf-layout/v2 manifest for manifest-backed partial loading
• a short usage note in mkdocs/docs/binaries.md

Why:

• the manifest consumer/runtime side is already in Zelph
• the missing piece for large hosted artifacts is a small producer tool that does not require the full ITIR repo
• this is the minimal slice Stefan can run directly after .index-file

Validation:

• built zelph from the current branch base
• smoke-tested tools/emit_zelph_hf_v2.py against a synthetic .bin + .index.json and verified shard files plus manifest emission

Usage:

python tools/emit_zelph_hf_v2.py \
  --bin wikidata-20260309-all.bin \
  --index wikidata-20260309-all-index.json \
  --output wikidata-20260309-all.hf-v2.json \
  --artifact-name wikidata-20260309-all \
  --hf-root hf://datasets/<owner>/<dataset> \
  --shard-root wikidata-20260309-all-shards

This produces:

• wikidata-20260309-all.hf-v2.json
• wikidata-20260309-all-shards/

After upload, the manifest can be consumed via .load-partial manifest.json ....