flatten blocklog Stats maps and add the README videos

[gerardrecinto]

Two changes.

README: the full demo and the short now show as clickable YouTube thumbnails under the demo GIF.

blocklog: Stats built a nested map for every rule and trace pair. It now uses flat maps with the per-run counts held by value. Same results, and the existing blocklog tests pass with -race. BenchmarkStats on the 5000-entry log, before and after:

time 1.97 ms to 1.42 ms
allocs 10095 to 128
memory 3.2 MB to 2.66 MB

verify was already at about 40 ns with 0 allocs on the safety check, so I left it alone.

[github-actions]

Gemini PR Review

Reviewed commit: 572311c7743cf3e8f033d1700423c488bb1a4411
Verdict: PASS

• README.md: The README.md changes are cosmetic, adding YouTube video embeds. This does not introduce any security or correctness issues into the codebase.
• tools/blocklog/blocklog.go: The changes to the Stats method are a refactor of the logic for tracking blocked and recovered runs. The new logic, using blockKey and perRun maps, appears functionally equivalent to the original nested map approach for calculating BlockedRuns and RecoveredRuns. It correctly counts distinct blocked steps within a rule run and distinct recoveries, and then aggregates whether all steps in a run recovered. The method remains protected by a mutex (l.mu.Lock()/Unlock()), preventing race conditions. No new security vulnerabilities (injection, unsafe deserialization, path handling, secrets, authorization), correctness bugs, leaks, or unhandled edge cases are introduced by this refactoring.