test(derive): close first mutation shard

Create shard output parents, exercise exact handler placement/signature/codegen contracts, preserve generated-file and form-bound diagnostics, and fail closed on malformed form symbol manifests. Public shard 1/8 passes through both 16-way partitions.

req: derive_handler/001

req: client_local/001

req: form/004

req: diagnostics/003

req: test/022

req: test/023

PLAN.md

@@ -21,7 +21,7 @@
 - [ ] **State:** In progress — the package-native capped xtask entry point is reachable, rejects unknown packages and invalid shards, propagates mutest failure, and mutation-tests `hemx-axum`, `hemx-build`, `hemx-core`, `hemx-js`, and the full `hemx-test` package cleanly; full package closure remains.
 - **User value:** maintainers can run one bounded repository command and trust that meaningful Rust logic across every mutation-applicable library is either killed or explicitly justified.
 - **Build:** add a capped `hemx-xtask` mutation command that invokes `/opt/repositories/mutest`/`mutest` through package-native test targets rather than the broken workspace-wide example path; enumerate only current mutation-applicable library/proc-macro packages; finish adversarial tests or simplify code until every survivor is classified; keep equivalent, invariant-only, and infrastructure-inapplicable classifications inspectable and minimal; document the exact local release command in the existing readiness surface.
-- **Blocked by:** none; broad survivors currently remain in `hemx-derive` and `hemx-lsp` outside already-clean focused contracts. The mutation entry point accepts validated one-based `SHARD/TOTAL` operands, maps them to native zero-based shards, uses shard-specific output directories, grants repo-owned compiler probes a 120-second floor, and preserves the unsharded gate. All eight deterministic `hemx-build` shards now pass: 1,304 mutants total, 1,070 caught and 234 unviable, including final shard `8/8` with 159 mutants (139 caught, 20 unviable). The complete 470-mutant `hemx-axum` package gate passes with 262 caught and 208 unviable after public page/form/multipart/registry/response/runtime proofs and narrow classification of infallible header parsing and streamed multipart unwrap-equivalent mutants.
+- **Blocked by:** none; broad survivors currently remain in `hemx-derive` and `hemx-lsp` outside already-clean focused contracts. The xtask mutation runner now creates shard output parents before invoking mutest, fixing first-use failure for newly sharded packages. `hemx-derive` public shard `1/8` is clean through deterministic partitions `1/16` (31 mutants: 24 caught, 7 unviable) and `9/16` (31 mutants: 28 caught, 3 unviable) after exact client-handler mode/signature/codegen, generated-file diagnostics, form generic bounds, and malformed symbol-manifest coverage; shards `2/8` through `8/8` remain. The mutation entry point accepts validated one-based `SHARD/TOTAL` operands, maps them to native zero-based shards, uses shard-specific output directories, grants repo-owned compiler probes a 120-second floor, and preserves the unsharded gate. All eight deterministic `hemx-build` shards now pass: 1,304 mutants total, 1,070 caught and 234 unviable, including final shard `8/8` with 159 mutants (139 caught, 20 unviable). The complete 470-mutant `hemx-axum` package gate passes with 262 caught and 208 unviable after public page/form/multipart/registry/response/runtime proofs and narrow classification of infallible header parsing and streamed multipart unwrap-equivalent mutants.
 - **Proof:** the new xtask mutation command exits zero within its documented bound, covers each applicable package, emits no unexplained missed mutant, and a deliberate adjacent mutation makes it fail. `cargo run -p hemx-xtask -- test` remains green. req: test/020 req: test/021
 
 ## 3. Elect and enforce the release license policy