Add a demo contract in `examples/` that triggers all built-in checks

[lonerthefirst3-sudo]

Summary

Add an examples/all-findings/ directory containing a single Soroban contract that intentionally triggers at least one finding for every check registered in default_checks().

Purpose

• Acts as a living integration test that all checks are firing
• Useful for demo / documentation screenshots
• New contributors can run soroban-guard scan examples/all-findings to see what the output looks like with real findings

What to add

1. examples/all-findings/Cargo.toml — standard Soroban fixture layout
2. examples/all-findings/src/lib.rs — annotated contract with each vulnerability pattern
3. Comment above each pattern explaining which check it triggers

Acceptance criteria

• soroban-guard scan examples/all-findings produces at least one finding per registered check
• Each finding maps to a clearly commented code pattern