docs: document fund() griefing fix and sponsor whitelist in README

[Wilfred007]

• Add 'Sponsor whitelist' subsection to contracts/escrow explaining the griefing vector (predictable issue_id + open fund() = squatting), the register_sponsor → fund two-step flow, and why admin-pre-registration was chosen over commit-reveal or permissionless recovery alternatives.
• Include a Soroban transaction-ordering note: no public mempool, but proactive squatting (not front-running) is the real threat model, and the whitelist closes it unconditionally.
• Update the function-list block with register_sponsor, force_reopen, get_registered_sponsor; add descriptions for all three new entrypoints.
• Update the Security model section with a dedicated 'fund() sponsor whitelist' bullet explaining NotWhitelisted and force_reopen.
• Update the Data models section to document DataKey::SponsorWhitelist.
• Update the Backend integration section: step 1 now describes the register_sponsor → verify → fund prompt sequence mergefi-backend must follow for escrow fund flows.

Summary

Closes #1
Closes #21
Closes #2
Closes #23

Related issues

Changes

Test plan

• Existing tests pass (make test)
• New tests added for changed behaviour
• Manually verified against a local node / testnet where applicable

Checklist

• Branch is up to date with main
• No debug / dead code left in
• cargo fmt and cargo clippy are clean

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