Add demo fixtures and command line interface

[heath-zulu]

Adds synthetic demo fixtures for the S3 and EC2 checks, a fixture-backed
loader (DemoS3/DemoEc2) that exposes exactly the methods the checks call,
and the posture CLI (main/run) that ties run_all and render together for
both demo and live AWS runs.

Demo mode drives the real check code through fixture-backed clients rather
than a parallel implementation, so what it demonstrates is the same code a
live run executes. Demo metadata carries no timestamp, which keeps the
whole demo output reproducible byte for byte.

All fixture data is synthetic: obviously fake account ids, example.com-style
bucket names, and RFC 5737 documentation-range IP addresses. No real account
data appears anywhere.

Test plan:

• pytest (33 passed, including 5 new CLI tests)
• ruff check . (clean)
• posture --demo and posture --demo --json run via the console entry point