Test robustness: gaps the 97%+ statement coverage hides

[MasterOfBinary]

From a test-quality audit (validated @ 58cee73; statement coverage is excellent but several behaviors are unguarded):

• Dynamic-config mid-run pickup has no deterministic unit test. The only behavioral guard was Example_dynamicConfig's exact output — which PR test: make Example_* output deterministic #69 (correctly) makes stop asserting batch sizes. After test: make Example_* output deterministic #69, a regression where doProcessors caches config.Get() once would pass the whole suite. Add a unit test that pins per-batch config re-read using a synchronization barrier (not sleeps).
• SourceError.Error() has 0% coverage — the "source error: %v" format contract is unguarded.
• The MaxItems < MinItems clamp in fixConfig is mutation-inert — deleting it fails no test (the MaxItems return path fires before MinItems is consulted). Cover it via observable batch sizes, or fold the clamp into validation (Resource limits + configuration validation #73).
• Change-detector tests in buffer_config_test.go assert DefaultItemBufferSize == 100 etc. — they restate constants.go and fail legitimate retuning; keep only the >= 1 sanity checks.
• Duplicate test helpers: testSource/countProcessor are defined twice with different shapes (external batch_test vs internal helpers_test) — drift hazard, consolidate.
• Uncovered branches: IgnoreErrors(nil) guard; ExecuteBatches with cfg.B == nil (only cfg == nil is tested); empty configs....

Sequencing: after PR #69 lands (it rewrites the example files these gaps interact with).