Add CLAUDE.md pointing at the practice that already exists

CLAUDE.md

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+# ProbabilisticDataStructures
+
+Probabilistic data structures for C#. Managed IL throughout — no `unsafe`, no pointers,
+no hardware intrinsics — targeting `net10.0` with one dependency, `System.IO.Hashing`.
+
+## Read these before writing anything
+
+- **[TESTING.md](TESTING.md) governs test work.** Read it before adding or changing a
+  test rather than after. It is the longest document here because testing is the hard
+  part of this repository, and most of it was learned by getting it wrong first.
+- **[CONTRIBUTING.md](CONTRIBUTING.md)** has the build and verify commands and the short
+  form of the same rules.
+- **[FORMAT.md](FORMAT.md)** specifies the persistence byte layout. It is held to the
+  code by `TestFormatDocumentation`, so it fails loudly rather than drifting quietly.
+
+## Verifying a change
+
+CONTRIBUTING.md has the commands. Two traps worth naming here, because both have
+produced a confidently reported result that was measured against the wrong binary:
+
+- **Read the build's exit code, unpiped.** Analyzers do not run on a project MSBuild
+  thinks is up to date, so an incremental `-warnaserror` build can pass on code that
+  will not compile from scratch.
+- **`dotnet test --no-build` silently reuses the previous binary.** If the build failed,
+  the test result you are reading belongs to the last thing that compiled. Confirm the
+  build exited zero *before* reading any test output.
+
+## Mutation
+
+Use `scripts/mutate.sh` rather than editing by hand. It restores the tree, applies one
+edit, builds, runs the tests and prints a verdict, and it refuses to start on a dirty
+tree — because the restore is `git checkout --`, which discards every uncommitted change
+in the target directory and not only the deliberate break. **Commit before mutating.**
+
+A test is not finished until it has been watched failing for the reason it names.
+
+## Rosters
+
+A sweep named "every X" must derive its roster from something a new case cannot omit
+itself from — the `StructureId` enum, or reflection over the public surface. A
+hand-written list means "every X someone remembered", and nothing fails when the next
+structure is added. Exemptions are written claims with a stated reason, and the reason is
+verified: a stale exemption fails as loudly as a gap. See TESTING.md.
+
+For the same reason, do not put a count in a comment or a doc. It is a roster maintained
+by hand, and one here was wrong for several releases before anyone read it closely.
+
+## Workflow
+
+Branch, open a PR, wait for CI, and **wait to be asked before merging**. Never push,
+merge, or tag unprompted. Scan the branch diff for secrets before pushing.
+
+CI runs the suite on five runners and all five gate merges: x64 and arm64 on both Linux
+and Windows, plus arm64 macOS. The arm64 labels pin an image version because there is no
+`-latest` form of them, so they need bumping by hand when an image is retired.
+
+## Layout
+
+```
+ProbabilisticDataStructures/      the library
+TestProbabilisticDataStructures/  the suite; fixtures/ holds stored payloads
+Benchmarks/                       BenchmarkDotNet
+scripts/mutate.sh                 step 3 of the TESTING.md loop
+```
+
+Tests see `internal` members: `Defaults.cs` declares `InternalsVisibleTo`.
+
+## Measuring on macOS
+
+BSD `grep` does not handle `\b` reliably, and substring matches corrupt naive coverage
+scans — `BloomFilter` matches inside `BloomFilter64`. Use a real parser for anything you
+intend to report a number from.