docs(security): add comprehensive privacy-pool threat model

[rbxict]

Summary

• replace the placeholder threat-model files with a full repository-backed threat model at docs/THREAT_MODEL.md
• document the current trust assumptions, security goals, threat actors, assets, and incident-response path for the Soroban privacy pool
• enumerate 24 concrete attack vectors with mitigations and residual risks, and add direct references to the current circuit and contract code paths that reviewers should inspect first

Validation

• verified the document now covers 24 attack vectors and 2,000+ words (attacks 24, words 2178)
• git diff --check

Payout Details

• EVM: 0xe744f6791a685b0A0cC316ED44375B69361c837F
• SOL: 8BsByR6rPqxDPku6dYtdoiSk6bdgE9YETbLQF2RGSw1C
• RTC: RTCfe4525ac631c325867a65d1b52b793779731d0d7

/claim #19
/payout EVM: 0xe744f6791a685b0A0cC316ED44375B69361c837F
/payout SOL: 8BsByR6rPqxDPku6dYtdoiSk6bdgE9YETbLQF2RGSw1C
/payout RTC: RTCfe4525ac631c325867a65d1b52b793779731d0d7

Closes #19
PoA: poa_1a67dc0bed375cc8

[rbxict]

I pushed a substantive follow-up here because the original branch only added placeholder threat-model files at the repo root and did not really satisfy the bounty scope. The current head replaces that with a full docs/THREAT_MODEL.md tied to the actual contract/circuit paths, covering 24 attack vectors, mitigations, residual risks, audit priorities, and incident response. I also updated the README link so the document is discoverable from the main security section.