Persist per-investor escrow storage keys

[Hillzo]

closes #253

Summary

• Move InvestorContribution, InvestorEffectiveYield, InvestorClaimNotBefore, and InvestorClaimed from instance storage to persistent storage.
• Keep caller-facing read/write semantics unchanged, including defaults for absent keys.
• Bump SCHEMA_VERSION to 6 and document the redeploy requirement because instance keys cannot be enumerated for an in-place migration.
• Update storage documentation and tests for the new persistent per-investor key behavior.

Security and correctness

• Auth boundaries remain unchanged.
• Contribution arithmetic still uses checked operations.
• Per-investor claim markers remain idempotent and address-scoped.
• Per-address persistent TTL can be extended through bump_ttl when investor addresses are supplied.

Validation

• cargo test
• Result: 392 passed; 0 failed; 11 ignored; doc-tests passed.

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