examples: the rule-check demo behind the clip

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The script behind the rule-check clip, on the shipped dispatch example, so anyone can reproduce every number in it.

examples/rule-check/demo.py (Python 3.9 standard library; PROBBIT_BIN or probbit on PATH; probbit 0.8.2 or later; uses only persona prove, live and live verify)

• Derives the "before" policy by text: examples/persona/dispatch-policy.yaml minus its routine_acts habit (the list item and the comment above it), written to a temp dir.
• 20 individuals (seeds 0-19): the day-0 rule check (persona prove --props dispatch-props.json, rule routine_work_acts), a day-0 test of 10 routine requests, 300 synthetic events in which users praise asking and criticise acting (live, fixed clock, src: human:synthetic), 20 routine requests on day 30, and live verify on every strand.
• The population line over seeds 0-99, then the shipped policy (habit included) with the same check and the same events.
• Flags --seeds 0-19, --events 300, --population 0-99. The output is identical between runs apart from the elapsed time on the last line.

Measured with probbit 0.8.2 (and with a build of main; same output):

day-0 test passed 17/20; by day 30, 4 of those ask about routine work every time.
rule check flagged 7/20 on day 0; drifted 7/20; flagged-and-drifted 7; drifted-but-not-flagged 0; flagged-not-drifted 0.
all strands replay byte for byte: True
population, seeds 0-99: flagged 24, drifted 24, both 24; drifted-but-not-flagged 0; passed day-0 test then drifted 15.
after one added habit: check says held_by_construction for 20/20; drifted 0/20; 10.1 s

Also: examples/rule-check/README.md; one README paragraph in "Check in CI" ("Reproduce the clip") with these numbers; a CHANGELOG 0.8.3 Added line; python/test_rule_check_demo.py, a smoke test that runs the demo with --seeds 0-3 --events 20 (about 2 s). Python suite: 45 OK. No Rust or workflow change; no tag, release or publish.