feat(escrow): add governed params setter wiring readiness

[BigMick03]

Closes #323

Summary
This PR adds an admin-gated governance parameter setter to the escrow contract in contracts/escrow. It introduces set_governed_params(admin, protocol_fee_bps, max_escrow_total_stroops), persists governed parameters, and flips the readiness checklist flag ReadinessChecklist.governed_params_set to true.

Files changed

• contracts/escrow/src/governance.rs
• contracts/escrow/src/lib.rs
• contracts/escrow/src/types.rs
• contracts/escrow/src/test/mainnet_readiness.rs
• contracts/escrow/src/test/mod.rs
• docs/escrow/mainnet-readiness.md

What this change does

• Adds GovernedParameters storage and DataKey::GovernedParameters
• Adds a secure admin-controlled setter for governed parameters
• Enforces validation of protocol_fee_bps and max_escrow_total_stroops
• Ensures governed_params_set becomes true only after a successful admin-set operation
• Exposes get_governed_parameters() for read-back
• Updates readiness documentation to match the new setter flow

Security and behavior notes

• set_governed_params requires initialize() to have been called and authenticated as the registered admin.
• Invalid parameter updates and unauthorized callers do not mutate readiness state.
• The readiness checklist update is atomic and persisted together with the governed parameter values.

Testing

Run the following commands from the repo root:

cargo fmt --all -- --check
cargo test -p escrow

Branch and commit guidance

• Branch name: feature/governed-params-setter
• Example commit message: feat(escrow): add governed params setter wiring readiness

Additional notes

• This PR is scoped to the TalentTrust escrow Soroban contract.
• Documentation in docs/escrow/mainnet-readiness.md has been aligned with the new contract behavior.

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