Add reproducible terminal demo generation script

[tayfuryldz]

Context

docs/headerproof-demo.cast is part of the documented CLI/output contract and is parsed by tests, but there is no reproducible command/script for regenerating it. That makes the recording easy to leave stale after an intentional CLI change.

Scope

Add a small deterministic local generation path for the existing controlled demo. Do not introduce a live target or change scanner behavior.

Start with:

• docs/headerproof-demo.cast — current asciinema v2 artifact
• tests/test_docs_contract.py::test_terminal_demo_is_valid_local_asciinema_contract
• docs/PROJECT_PROOF.md — rules separating the localhost demo from a real finding

A focused script under scripts/ or an equivalently small documented command is appropriate. Discuss first if the solution needs new runtime dependencies.

Expected outcome

A maintainer should be able to regenerate the demo from a controlled localhost fixture and get a valid asciinema v2 cast that preserves the documented compact finding shape.

Acceptance criteria

• Generation uses only a controlled localhost fixture; no public target is contacted.
• Output is valid asciinema v2 and remains parseable by the existing docs contract test.
• The generated transcript contains the HeaderProof localhost command and at least one expected compact finding line.
• Re-running the generator does not introduce meaningless/random documentation churn; normalize or control unstable values where practical.
• docs/PROJECT_PROOF.md documents the exact regeneration command.
• No scanner behavior changes and no new dependency unless discussed first.

Validation

Run at minimum:

python -m pytest -q tests/test_docs_contract.py

Also run the new generator twice and inspect the diff so avoidable nondeterministic churn is caught before opening the PR.

Comment before starting so a maintainer can confirm the scope is still available.

[tayfuryldz]

Maintainer note: this issue is currently available for a new contributor. The scope and acceptance criteria above are current. If you want to take it, leave a short comment before starting; I will confirm ownership and review a focused PR promptly.