Reject malformed honeypot timestamps

[nickmoreton]

Summary

• Treat empty, non-numeric, and otherwise non-convertible honeypot timestamps as failed checks instead of allowing request-processing exceptions.
• Add focused method tests and full Wagtail form-flow regression coverage.
• Keep the copyable developer-guide implementation aligned with the guarded conversion, document the design and implementation plan, add the required Unreleased changelog entry, and ignore local .worktrees/.

Root cause

time_diff() converted the untrusted timestamp POST value directly with int(value). A present but empty or non-numeric field therefore raised ValueError (or TypeError for non-string invalid inputs) rather than following the fail-closed behavior already used for missing honeypot fields.

Impact

Malformed submissions are now silently ignored, continue to receive the normal thank-you response, and do not create a form submission. Valid timestamps, interval boundaries, settings, and template output are unchanged.

Closes #60.

Verification

• uv run manage.py test tests.test_methods tests.test_form — 10 tests passed
• make test — 17 tests passed; 100% package coverage
• make lint — Ruff passed
• git diff --check — passed