Add waitlist contract example

[alexgduarte]

Contributes to #11

Summary

• Add a new waitlist simple contract example.
• Store one active waitlist entry per address with monotonic first-come queue positions.
• Let the owner admit or remove waitlist members while members can update notes or leave the waitlist.
• Track waiting, admitted, active, and next-position stats.
• Add owner transfer, configurable note and capacity limits, entry queries, stats queries, config queries, and paginated entry-list queries.
• Include README documentation, root README navigation, schema generation, and nine focused unit tests.

Validation

• cargo fmt --manifest-path waitlist/Cargo.toml -- --check
• cargo test --manifest-path waitlist/Cargo.toml --target-dir /private/tmp/codex-waitlist-target
• git diff --check