docs(factory): document DataKey storage layout

[vku2018]

Summary

• add a Storage Layout section to docs/factory.md covering all 10 DataKey variants
• document storage tier, key encoding, value type, TTL constants, bump triggers, and source line references
• call out the current Allowlist(Address) persistent TTL gap and the read/single-create TTL gaps
• add indexer reconstruction guidance for policy state, stream IDs, allowlist membership, and rate-bound events

Closes #827

Validation

• PASS: all 10 requested DataKey variants are present in docs/factory.md
• PASS: allowlist TTL gap note is present
• PASS: git diff --check
• BLOCKED: cargo test -p fluxora_factory could not be run because cargo is not installed in this Windows environment.