docs(contracts): add group treasury multisig and authorization model conceptual documentation

[cythecode]

Overview

Adds comprehensive conceptual documentation for the group_treasury Soroban smart contract's authorization and multi-signature model ($M$-of-$N$) in contracts/docs/concepts-treasury-multisig-model.md.

Related Issue

Closes #471

Changes

Smart Contract Documentation

• [ADD] contracts/docs/concepts-treasury-multisig-model.md
  • Explains system roles and authorization architecture (Admin vs. Members vs. Public).
  • Details membership lifecycle management (add_member, remove_member, is_member, get_members).
  • Describes the threshold approval model ($M$-of-$N$), initialization requirements, and immutability.
  • Details the full withdrawal proposal lifecycle (propose_withdraw, approve_withdraw, reject_withdraw).
  • Details the mathematical blocking minority formula ($\text{rejections} \ge \text{member_count} - \text{threshold} + 1$) for early rejection termination.
  • Documents security guarantees (collusion resistance, double-voting prevention, event auditability) and known edge cases/limitations.
  • Provides Mermaid state machine and sequence diagrams.
  • Cross-references api-group-treasury.md and related contracts.

Verification Results

$ cargo test
running 46 tests
test result: ok. 40 passed; 0 failed; 6 ignored; 0 measured; 0 filtered out; finished in 0.42s

running 18 tests
test result: ok. 18 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.42s

running 8 tests
test result: ok. 8 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.04s

Acceptance Criteria	Status
Explains threshold model in plain terms	✅ Fully documented with examples and initialization rules
Explains security properties and known limitations	✅ Collusion resistance, blocking minority, edge cases covered
Cross-references API doc (api-group-treasury.md)	✅ Linked with context to related docs and source files

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[github-actions]

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