lib: CLAUDE.md documents bootstrap-minimalism subset stance

lib/CLAUDE.md

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+lib/ — Hare-shaped standard library, deliberately a subset.
+
+Scope: every `lib/*` directory except `lib/ww/`, which is the compiler
+frontend port and has its own rules (see `lib/ww/CLAUDE.md`).
+
+Names mirror Hare. Before adding a function, find its counterpart in
+`ref/hare/<module>/` and copy the name verbatim (`strings.compare`,
+`bytes.index`, `strconv.stoi64`, `io.read`, `path.init`). Don't
+invent. Don't shorten. Don't reorder parameters.
+
+Signatures are intentionally a Plan 9 / pre-tagged-union subset,
+because the wwstage compiler doesn't yet have ergonomic tagged-union
+returns or full variadic ABI:
+
+- `i32` with `-1` sentinel stands in for Hare's `(size | void)` and
+  `(size | io::EOF)`. Example: `bytes.indexbyte(s, c) i32` returning
+  `-1` for not-found.
+- Plain `str` errors stand in for tagged error types (`invalid`,
+  `overflow`, `io::error`). Example: `strconv.parse64(s) (i64 | str)`
+  where the `str` is the message Hare would have tagged.
+- `lib/fmt` is print/println-only — printf-family waits on a variadic
+  ABI.
+- Allocation-free at the boundary: where Hare returns a freshly-
+  allocated slice, take a caller-supplied buffer. No GC, and the `rt`
+  allocator is not a stable public surface yet.
+
+Don't ship a richer surface than Hare has. A documented subset is
+fine; an extension, rename, or convenience-wrapper is not — return
+shapes will change as the compiler gains tagged unions, variadics,
+and a stable `rt` allocator, and callers should not bake the current
+subset shape into themselves.
+
+When the compiler can express the full Hare signature, graduate the
+module — replace the `-1` / `str` shape with the tagged-union shape
+in one go and fix the callers. Don't keep both around.