fix(shell): decode Windows output reliably

[h3c-hexin]

Background

This continues #24 from a Pinvou-owned branch because the original head repository is no longer available for follow-up commits or workflow dispatch. The first commit preserves @zhuowp as author, retains the original DCO trailer, and has a tree identical to #24 head 22ec868291922aa133197dd9736a0da31ae18329. Follow-up commits resolve the Clippy finding and add executable Windows coverage.

Windows-native tools may emit stdout or stderr using the active ANSI code page even when the surrounding shell is configured for UTF-8. The previous lossy, per-poll UTF-8 decoding could therefore produce mojibake and could corrupt a multibyte character split across background polls.

The reusable fix was first contributed upstream in Hmbown/CodeWhale #5602.

Changes

• preserve strict UTF-8 decoder state across shell reads
• fall back to the current Windows ACP only after genuine invalid UTF-8
• share decoding semantics across synchronous output, snapshots, raw deltas, tails, and detached readers
• seal detached streams without discarding bytes captured before the cutoff
• derive completion events and evidence from one output cutoff
• add decoder, shell-lifecycle, and forkguard_* regression coverage
• remove a behavior-neutral loop whose branches all returned, resolving clippy::never_loop
• make the incomplete UTF-8 unit test independent of the host ACP
• run focused shell regressions on windows-latest in Pinvou fork CI
• rerun the required contribution gate after PR head updates

Dependency advisory remediation remains separate in #27 because #24 does not change Cargo.lock.

No-Issue: continues #24 from a maintainable repository-owned branch while preserving contributor credit.

Testing

• cargo fmt --all -- --check
• git diff --check
• cargo test -p codewhale-tui --lib tools::shell --locked locally (110 passed)
• cargo test -p codewhale-tui --lib tools::shell::output::tests --locked locally after the Windows test-fixture fix (9 passed)
• cargo clippy -p codewhale-tui --lib --locked --no-deps -- -A warnings -D clippy::never_loop
• Exact original-commit tree comparison against fix(shell): decode Windows output reliably #24 head
• Final-head Pinvou fork CI check on Linux (run 32929782904)
• Final-head native windows-latest shell regressions (run 32929782904)
• Full workspace Clippy (the public r10 baseline has unrelated Rust 1.98 warnings under -D warnings)

Risks

ACP detection is deliberately a fallback after invalid UTF-8 and cannot infer OEM or arbitrary child-selected encodings. The change is confined to the shell byte-decoding lifecycle and does not modify sandbox, PowerShell, Python, or command-generation behavior.

The contribution gate previously ran only when a PR was opened or reopened, so strict branch protection lost the required context after every update. This branch adds the missing synchronize event. The current PR still needs its final gate regenerated after #27 lands and the combined tree is updated.

Advisory checks remain red until the independent lockfile fix in #27 lands; #28 must be evaluated again against the resulting combined tree before merge.

Checklist

• Updated docs or comments as needed
• Added or updated tests where relevant
• Verified TUI behavior manually if UI changes (not a UI change)
• Harvested/co-authored credit uses a GitHub numeric noreply address (not applicable; the original signed commit is preserved)

[h3c-hexin]

Temporarily closing while rebasing onto the newly merged #27 baseline. This avoids duplicate CI runs and lets the required contribution gate attach to the final head when the PR is reopened.

[h3c-hexin]

Rebased onto pinvou3-clean after #27 merged. Reopening at final head 309cafb58d05969a0128c3d6aef7bb13fe0e395e to run the required gate, security audits, Linux checks, and native Windows shell regressions against the combined tree.