src: make compile cache zstd dictionary reproducible#17
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Add tools/train_compile_cache_dict.mjs, a maintainer tool that regenerates src/compile_cache_zstd.dict end to end, and replace the embedded dictionary with its byte-for-byte reproducible output. The script walks a fixed in-tree corpus, harvests a V8 code cache from each module via vm.compileFunction with produceCachedData (the same shape the CommonJS loader produces at runtime), feeds the blobs to `zstd --train`, and writes the 16 KiB dictionary in place. The output is byte-for-byte stable when node is run with --predictable (the script re-execs itself with it, since V8 otherwise randomizes the string hash seed and that seed leaks into cachedData), the corpus and its sorted order are fixed, and the node build is fixed. This documents and makes reproducible exactly how the embedded dictionary was created, so a future maintainer can regenerate it (e.g. after a V8 or corpus change) and review the resulting diff. Refs: nodejs#63861
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