Add STDLIB.md

Signed-off-by: Matthias J. Kannwischer <matthias@kannwischer.eu>

Author: mkannwischer

STDLIB.md

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+[//]: # (SPDX-License-Identifier: CC-BY-4.0)
+
+# Standard Library Dependencies
+
+mldsa-native has minimal dependencies on the C standard library. This document lists all stdlib functions used and configuration options for custom replacements.
+
+## Dependencies
+
+### Memory Functions
+- **memcpy**: Used extensively for copying data structures, keys, and intermediate values (40+ occurrences)
+- **memset**: Used for zeroing state structures and buffers (3 occurrences). **Note**: This is NOT used for security-critical zeroing - that is handled by `mld_zeroize` which has its own custom replacement mechanism
+
+### Debug Functions (MLDSA_DEBUG builds only)
+- **fprintf**: Used in debug.c for error reporting to stderr
+- **exit**: Used in debug.c to terminate on assertion failures
+
+## Custom Replacements
+
+Custom replacements can be provided for memory functions using the configuration options in `mldsa/src/config.h`:
+
+### MLD_CONFIG_CUSTOM_MEMCPY
+Replaces all `memcpy` calls with a custom implementation. When enabled, you must define a `mld_memcpy` function with the same signature as the standard `memcpy`.
+
+### MLD_CONFIG_CUSTOM_MEMSET
+Replaces all `memset` calls with a custom implementation. When enabled, you must define a `mld_memset` function with the same signature as the standard `memset`.
+
+See the configuration examples in `mldsa/src/config.h` and test configurations in `test/custom_*_config.h` for usage examples and implementation requirements.