[skyswordw]

Stow could list Zstd-coded 7Z archives but could not verify, preview or extract their payloads. This builds pinned official 7-Zip 26.04 source with a small decoder-only registration patch for method 04F71101, reusing the upstream Zstd decoder. It adds no second parser, external plugin or full codec fork.

Closes #316.

The registered 7Z decoder accepts established current/legacy properties, rejects unknown flags/lengths and caps each window at 512 MiB before allocation. This is not a total-process memory limit. Standalone .zst/.tar.zst retains upstream behavior; no Zstd 7Z encoder is added. Staged Save add/rename/delete works for the tested solid/non-solid Zstd inputs.

The executable identifies itself as 26.04-stow-zstd1. Source checksum and patch/build-recipe hashes are pinned. Both engine bundles contain complete modified corresponding source, licenses and a tested offline Universal 2 build script; package checks inspect archive contents. XPC contracts, password-on-stdin, inherited sandbox and destination containment/publication stay in place. Unified patch context whitespace is explicitly preserved by .gitattributes.

Validation:

• Final integration with main/Report exFAT free space accurately when important-usage capacity is zero #341: 771 tests passed, zero failures/skips; all 1,291 zh-Hans strings translated, generated plists unchanged, diff checks passed. No new product strings.
• Exact bytes through sandboxed verify/preview/selected/all extraction: ordinary, Delta, encrypted/split, current/legacy properties, solid/non-solid multi-file and frame-checksummed inputs. Positive split cases preserve every source part.
• Unknown properties, excessive window, entry/frame CRC corruption and truncation fail without publication. Wrong passwords retain typed errors. A 2 GiB expansion is cancelled only after intermediate progress; no output is published.
• Offline source rebuild succeeded. Native arm64 and Rosetta x86_64 exact-byte/negative probes passed; explicit Zstd 7Z encoding remains unsupported.
• Ad-hoc Release structural/signature/DMG validation: 12 Universal 2 binaries, zero problems, both source/license copies verified.
• Local launch-smoke limitation: candidate and unchanged baseline both exit without a fresh Welcome PNG in this local launch context. No UI or smoke gate was weakened. Clean CI package/smoke and snapshots must pass before merge. Developer ID/notarization acceptance remains with release validation.

Codex completed functional/security/test review; Claude Code retains visual/motion ownership. No new physical Finder or VoiceOver acceptance is claimed. #317 stays open for its separate FSKit exFAT publication failure; #341 only fixes its capacity misdiagnosis.

Evidence, provenance, commands and release limits: Handoff/zstd-7z-decoder-316.md. The small upstream patch is explicit maintenance debt to review on each 7-Zip update.

[skyswordw]

No blocking functional findings remain. Parent review and an independent Terra read-only review checked the minimal official-source patch, decoder-only registration, state initialization, pre-allocation 512 MiB per-window cap, supported/unknown properties, cache invalidation and complete corresponding-source distribution. Review follow-ups strengthened active-cancellation evidence, split-source preservation, offline rebuilding and staged Save coverage.

Final local integration passed 771 tests with zero failures/skips, 1,291 complete zh-Hans translations and generated-plist/diff checks. Exact-byte verify/preview/extraction, encryption/Delta/splits, solid/non-solid multi-file input, corruption/unknown properties/over-window nonpublication, wrong passwords, intermediate-progress cancellation and Save mutations passed. Both native arm64 and Rosetta x86_64 decoder slices were exercised. Source was rebuilt offline; both bundled engine copies retain licenses/source and sandbox/signature checks.

All refreshed PR CI checks passed, including build-test, Universal 2, snapshots and Release package. The clean package log explicitly confirms zero ad-hoc verification problems and a freshly rendered Welcome window. This resolves the local-only baseline launch-smoke limitation as a merge blocker without weakening the gate. No Developer ID/notarization or physical Finder installation acceptance is claimed; normal release validation remains required.

#316 is resolved within the documented property/window limits. #317 stays open for its separate FSKit exFAT publication failure. Codex owns functional/security/test work; Claude Code retains visual/motion ownership and dedicated VoiceOver remains deferred in #93.