docs(changeset): add release notes

Author: gkoos

.changeset/large-deer-send.md

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+---
+'@gkoos/ffetch': minor
+---
+
+Added
+
+- Support for modern AbortSignal.timeout and AbortSignal.any APIs (requires polyfill for Node <20 and older browsers).
+- cause property to all custom error classes for better error provenance.
+
+Changed
+
+- Refactored timeout and abort logic to use only AbortSignal APIs; removed manual timeout fallback.
+- Tests now strictly assert error types and .cause properties.
+- Improved test coverage for edge cases and fallback branches.
+
+Docs
+
+- Updated README with new prerequisites, error .cause documentation, and polyfill instructions.