Route the configured default maximum window size through the Rust policy layer while keeping the build-time C configuration value and decoder layout in C. The explicit projection preserves overridden builds exactly and gives the scalar policy a focused null-input contract and tests.
Test Plan:
- git diff --check -- lib/decompress/zstd_decompress.c rust/src/zstd_decompress.rs
- capped serial cargo check, clippy, nightly fmt, C syntax checks, and focused default-window tests (passed in the worker)
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