Add explicit aggregate acquisition start policy

[Ashbrunaugh]

Summary

• add an explicit, opt-in concordant acquisition-start policy for aggregate KWW analysis
• preserve the released first-frame API default while requiring manifests to declare their start policy
• record selected start provenance and late-start counts in run, independent-unit, and condition outputs
• keep aggregate start selection separate from later upward-transient repair and reject double startup processing
• update the generic scientific contract, synthetic profile, and public snapshot checksums

Scientific audit

• evaluated 72 manuscript runs
• 70 retained frame 0; two pH 4.5 runs selected frame 1 at 0.600 and 0.883 min
• a 36-setting sensitivity grid selected the same two runs throughout
• pH 4.5 mean relaxation changed from 3.26 +/- 0.34 min to 3.06 +/- 0.19 min without changing condition ordering

Verification

• 44 public tests passed
• wheel and source distribution built successfully
• 85-file disclosure-clean public snapshot verified against the tracked checksum manifest
• independent Claude Code audit completed; required fail-closed and CLI-safety findings addressed