fix(cli): consolidate snapshot and frame diagnostics - #2402

Summary

• preserve exact explicit snapshot timestamps and append the readable tail only when needed
• fail video snapshot extraction clearly when FFmpeg is unavailable
• propagate diagnostic browser navigation timeouts consistently
• retain alpha in snapshot PNGs and create parent directories for motion-shot output
• preserve regression coverage from fix(snapshot): preserve exact requested times #2325, fix(cli): fail video snapshots without FFmpeg #2302, fix(cli): honor navigation timeout in diagnostics #2246, and fix(cli): preserve snapshot alpha and create shot dirs #2238

Verification

• CLI test suite: 1,739 passed, 2 skipped
• focused snapshot/frame diagnostics: 84 passed
• CLI typecheck
• diff whitespace check