nw-pbt-fundamentals

# PBT Fundamentals Core property-based testing knowledge, language-agnostic. Load language-specific skills for framework syntax. ## What PBT Is Instead of specifying individual test cases, define properties (rules for all valid inputs) and let the framework generate hundreds of random inputs to attempt falsification. On failure, framework automatically shrinks input to minimal reproducing case. PBT complements example-based tests. Use both. ## Property Types Taxonomy ### Decision Table: Choosing Property Types | Property Type | Use When | Example Domain | |--------------|----------|----------------| | Invariant | Output has structural guarantees regardless of input | sort preserves length, tree stays balanced | | Idempotency | Applying operation twice equals applying once | formatting, deduplication, HTTP PUT | | Round-trip | Two operations are inverses | encode/decode, serialize/deserialize, compress/decompress | | Oracle | Simpler reference implementation exists | optimized algo vs naive, new parser vs stdlib | | Metamorphic | Can't verify single output but can relate multiple outputs | ML classifiers, search engines, numeric computation | | Commutativity | Operation order shouldn't matter | set operations, independent migrations | | Inductive | Property holds for base case and each incremental step | recursive data structures, incremental builds | | Hard-to-find, easy-to-verify | Solution expensive to compute but cheap to check | pathfinding, factorization, compilat...