Hoist prime order thresholds in CPU heuristics

docs/ByDivisorPathMap.md

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+# --mersenne=bydivisor Path Map
+
+This temporary map tracks the code paths exercised by the `--mersenne=bydivisor` mode and notes the latest review status for each branch. Entries move from _Queued_ to _Reviewed_ once the surrounding loops have been audited for invariant work.
+
+## High-level Flow
+- `EvenPerfectBitScanner` CLI parsing → _Reviewed_
+- `MersenneNumberDivisorByDivisorTester.Run` → _Reviewed_
+- `EvenPerfectBitScanner.IO.CalculationResultsFile`
+  - `LoadCandidatesWithinRange` → _Reviewed_
+  - `EnumerateCandidates` → _Reviewed_
+- `IMersenneNumberDivisorByDivisorTester` implementations
+  - `MersenneNumberDivisorByDivisorCpuTester`
+    - `IsPrime` → _Reviewed_
+    - `CheckDivisors` (128-bit path) → _Reviewed_
+    - `CheckDivisors64` → _Reviewed_
+  - `MersenneNumberDivisorByDivisorGpuTester`
+    - `IsPrime` → _Reviewed_
+    - `CheckDivisors` → _Reviewed_
+    - `DivisorByDivisorKernels.CheckKernel` → _Reviewed_
+
+- `MersenneDivisorCycles`
+  - `TryCalculateCycleLengthForExponentCpu` → _Reviewed_
+  - `TryFactorIntoCountsInternal` → _Reviewed_
+  - `PollardRho64` → _Reviewed_
+  - `ReduceOrder` (includes `ProcessReduceOrderPrime`) → _Reviewed_
+- `PrimeOrderCalculator`
+  - `InitializeStartingOrderCpu` → _Reviewed_
+  - `ExponentLoweringCpu` / `ProcessExponentLoweringPrime` → _Reviewed_
+  - `ValidateOrderAgainstFactors` / `ValidateOrderForFactor` → _Reviewed_
+  - `TryConfirmCandidateCpu` / `CheckCandidateViolation` → _Reviewed_
+  - `TryHeuristicFinishCpu` / `BuildCandidates` / `SortCandidates` → _Reviewed_
+
+## Next Steps
+- Hold for additional guidance on further --mersenne=bydivisor audits; computation branches documented above are fully reviewed.
+- Defer result aggregation and reporting reviews until instructed otherwise, keeping the focus on computation-only branches.