tests: add replay harness comparator and unit tests

Implement pairwise comparison of normalized backend results with
field-level precision. Covers events (author, text, function calls,
responses, state deltas), state (deep recursive diff), summaries
(loss, overwrite, wrong-session-affiliation detection), and memory.
Includes AllowedDiff rules to suppress known-acceptable backend
differences via pattern matching.

30 unit tests verify: identical detection, count mismatches, field-
level diffs, event-index precision, nested state diff paths, summary
loss / overwrite / mis-affiliation, memory diffs, and allow-rule
suppression with backend-pair scoping.

Updates #89

RELEASE NOTES: NONE

Author: DNKYr