feat: round-13 abstractions — AggregateError for parallel fan-out

.hermes/plans/mytheclipse-round13-spec.md

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+# Implementation Spec: Round 13 — COMPLETE
+
+## New Feature
+
+### AggregateError (mytheclipse-core, resiliency)
+File: `crates/mytheclipse/src/aggregate_error.rs`
+- Collects multiple `E: std::error::Error` from parallel/fan-out tasks into one
+  error — natural failure type for `join_all` + batch/fan-out resilience
+- `empty()` / `with_context(..)` / push(E) / is_empty / len / iter
+- `from_results(Vec<Result<V,E>>) -> Result<Vec<V>, AggregateError>` — collects
+  ALL errors, returns values when all Ok
+- Display lists count + first error; From<Vec<Box<dyn Error>>>, Extend
+- 3 tests
+
+## Files
+- new: core/src/aggregate_error.rs
+- core/lib.rs: +module+export AggregateError (resiliency)
+
+Build: exit 0. Tests: 0 FAILED (97 core pass). Clippy: 0 new warnings.