Retroactive design + plan documentation for Phase B.3 (deferred
analysers requiring custom Analyser subclasses for cross-entry and
threshold logic). Records the architectural shift away from vanilla
PatternAnalyser, the threshold constant rationale (event-pairing /
sliding-window / consecutive-snapshot deltas), and the synthetic
fixture extensions that exercise both trigger and non-trigger paths.
Plan is as-built with checkboxes pre-checked and SHAs referenced.
