docs(io): make portable-v2 the project movement workflow (#862)

[DecisionNerd]

Summary

• Add a user-facing portable-v2 guide for preview, expanded/bundle export, full verification, atomic clean import, selective sharing, air-gapped transfer, and digest-pinned OCI promotion.
• Map the Rust API and CLI to the same Rust-owned workflow and document package/transport/source/import identity boundaries, integrity versus authenticity, credentials, and HTTPS policy.
• Explicitly prohibit raw live .graphforge copies and committing graph/source/package/materialized data to Git; publish the guide in site navigation and API reference.

Closes #862. #862 is a native sub-issue and blocker of #740 and remains blocked by #744 so the documented binding/CLI surface lands first.

Test plan

• pnpm docs:build (106 pages)
• pnpm --filter @curatelabs/graphforge-docs-site check-links (13,096 links, zero broken/stale)
• git diff --check

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[codspeed-hq]

Merging this PR will not alter performance

✅ 64 untouched benchmarks

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_{Comparing docs/862-portable-v2-movement (502b920) with main (c9f4dd8)}