Add multi-file usage examples for lookup, key rotation, and audit

[albonec]

Closes #367

Adds three new subcommands to the akd-examples binary, each as a multi-file module following the same pattern as the existing fixture_generator and mysql_demo examples:

• basic-lookup – publish a user pool in a single epoch and verify a client lookup proof
• key-rotation – simulate successive key rotations for one user and verify the complete history proof across all versions
• append-only-audit – populate several epochs and verify append-only integrity as an independent auditor

Each example is split across focused sub-files (setup.rs, client.rs, proofs.rs, etc.) for readability, and each ships with a tests/ sub-module that exercises both WhatsAppV1Configuration and ExperimentalConfiguration via the test_config! macro. All 179 tests pass (cargo test -p examples).

Test plan

• cargo build -p examples compiles cleanly
• cargo test -p examples — all 179 tests pass
• cargo run -p examples -- basic-lookup runs and prints a verified lookup
• cargo run -p examples -- key-rotation runs and prints a verified history table
• cargo run -p examples -- append-only-audit runs and prints a verified audit result
• cargo run -p examples -- basic-lookup --users 10 --label judy@example.com works
• cargo run -p examples -- key-rotation --rotations 10 works
• cargo run -p examples -- append-only-audit --epochs 8 works

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (3ce5335) to head (d47f386).
⚠️ Report is 35 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #487      +/-   ##
==========================================
+ Coverage   88.61%   89.31%   +0.70%     
==========================================
  Files          39       38       -1     
  Lines        9109     7442    -1667     
==========================================
- Hits         8072     6647    -1425     
+ Misses       1037      795     -242     

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