docs(zerobucket): add refactor notes and test summary

Author: Charles-Lee03

docs/refactor-zero-bucket-notes.md

@@ -0,0 +1,54 @@
+# Refactor Notes: ZeroBucket (Task 1)
+
+## Summary
+
+Converted implicit lazy loading (sentinel values Long.MAX_VALUE / Double.NaN) into explicit state flags (indexComputed, thresholdComputed). Added static factories (fromThreshold, fromIndexAndScale) while retaining the original public constructor (deprecated) and the original APIs (minimalEmpty, minimalWithCount, merge, collapseOverlappingBuckets\*, compareZeroThreshold). Added equals/hashCode/toString for value semantics and stronger testability.
+
+## Key Changes
+
+| Aspect            | Before                                  | After                                                                           |
+| ----------------- | --------------------------------------- | ------------------------------------------------------------------------------- |
+| Lazy tracking     | Sentinels (Long.MAX_VALUE / Double.NaN) | Booleans (indexComputed / thresholdComputed)                                    |
+| Constructors      | Public constructor only                 | + fromThreshold / fromIndexAndScale (constructor kept, deprecated)              |
+| API compatibility | merge, collapse\*, minimalWithCount     | Preserved (restored where removed)                                              |
+| Value semantics   | None                                    | equals, hashCode, toString                                                      |
+| Tests             | None for ZeroBucket                     | Added ZeroBucketTests (lazy, merge, minimal, invalid input, collapse, equality) |
+
+## Rationale
+
+Explicit flags improve readability and reduce mental overhead. Factories clarify intent (threshold-driven vs index-driven). Value semantics simplify reasoning in future merging logic and aid debugging (toString includes internal state).
+
+## Risks & Mitigation
+
+| Risk                                                             | Mitigation                                                                                              |
+| ---------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------- |
+| Off-by-one index computation regression                          | Preserved original +1 logic in index() lazy path                                                        |
+| Breaking callers using removed methods                           | Restored original methods (merge, minimalWithCount, collapseOverlappingBuckets\*, compareZeroThreshold) |
+| Hidden performance cost from forcing both computations in equals | Acceptable; equality rarely on hot path; keeps simplicity                                               |
+
+## Tests Added
+
+- testFromThresholdLazyIndex
+- testFromIndexLazyThreshold
+- testMinimalEmptySingleton
+- testMinimalWithCount
+- testMergeKeepsLargerThreshold
+- testInvalidNegativeThreshold
+- testEqualityAndHashStable
+- testCollapseNoOverlapReturnsSame
+- testToStringContainsKeyFields
+
+All passed with:
+`./gradlew :libs:exponential-histogram:test --tests "*ZeroBucketTests"`
+
+## Evidence Pointers
+
+- Commit(s): refactor(zerobucket)... (and fix if needed)
+- PR: refactor: ZeroBucket explicit lazy state & value semantics (Task 1)
+- Build screenshot: ZeroBucketTests BUILD SUCCESSFUL
+- Diff: removal of sentinel logic / introduction of flags & factories
+
+## Future Follow-up (Not in Task 1)
+
+- Remove deprecated constructor after downstream code migrates to factories.
+- Consider benchmarking overhead of toString in large diagnostics.