feat: add escrow immutability flow

[dev-jodee]

Summary

• add escrow-level mutability flag and a SetImmutable instruction to permanently lock config updates
• update processors so config-changing instructions require mutable escrow, while deposits remain allowed
• simplify initialization by using Escrow::new(..., is_mutable) and remove redundant immutability constructor paths
• update integration tests/fixtures for immutability behavior and remove unnecessary withdraw pre-step

Test Plan

• cargo fmt -p escrow-program -p tests-escrow-program --check
• cd program && cargo clippy --all-targets -- -D warnings
• cd tests && cargo clippy --all-targets -- -D warnings

Notes

• supersedes feat: add escrow immutability flow #29 (closed due polluted commit history)

[dev-jodee]

Already reviewed, had to close the other pr and reopen to simplify conflict resolution.