fix(managed-cli): map undecodable staged files to unverified#369

Summary

• A staged checksum or VERSION file that is not valid UTF-8 raised UnicodeDecodeError, which failure_status() degraded to the generic error instead of the unverified tampering signal used for other refused content.
• Decode failures in install() now fold into the existing checksum_file and version_file codes, so binary staged files report unverified like mismatched ones.
• Observable result: same install refusal, but the widget renders the tampering signal instead of a generic error.

Related issue

None

Documentation impact

• docs/development.md already documents that refused downloads report unverified; this change aligns the behavior with the doc, so no guide update is needed.
• TODO.md is unaffected: no parity entry covers helper failure mapping.
• Changelog: updated Unreleased.

Verification

• New test test_binary_staged_files_report_unverified in tests/test_managed_cli.py (binary .sha256 and binary VERSION fixtures): observed red before the fix (UnicodeDecodeError message mismatch), green after.
• make check: full suite green locally on the stacked tip 0e63e84. Skipped locally (tools absent, CI covers them): actionlint, pyflakes. QML hardening noted partial import/type checks locally ("CI requires full checks").
• CI: green on 0e63e84 — check, smoke, smoke-runtime, pr-title all pass (run).

Visual changes

None

Review notes

• Stacked on #368 (base fix/todo-gate-harden); retarget to main once #368 merges. This PR's own diff is only the managed_cli commit.
• Only UnicodeDecodeError is folded in; a missing staged file still reports error, which fits a local staging problem rather than refused content.