feat(pragmastat): add compare1 and compare2 options (#3591)

* feat(pragmastat): add --compare1 and --compare2 for confirmatory analysis

Leverage pragmastat v12.0.0's compare module to test one-sample (center/spread)
and two-sample (shift/ratio/disparity) estimates against user-defined thresholds,
producing a verdict of less/greater/inconclusive per metric.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor(pragmastat): deduplicate fallback paths and expand mutual exclusivity tests

Extract fallback closures in compute_compare1 and compute_compare2 to eliminate
repeated struct construction across error/empty paths. Add test cases for
--compare1 + --compare2 and --compare2 + --twosample mutual exclusivity.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(pragmastat): harden threshold parsing per Copilot review

Trim metric/value after colon split, reject NaN/Inf thresholds,
and add many-pairs warning to compare2 matching twosample behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: jqnatividad