fix(upgrade): exit non-zero with clear error when download fails

[Chemaclass]

Summary

bashunit upgrade reported success even when the binary download failed, leaving users on the old version without any clue what went wrong:

> bashunit upgrade
> Upgrading bashunit to latest version
Failed to download bashunit
> bashunit upgraded successfully to latest version 0.35.0
$ bashunit -v
bashunit - 0.34.1

This PR makes the command behave correctly and tells the user why it failed.

Behavior changes

• Aborts on download failure with a non-zero exit code (no more false success message).
• Prints the exact URL it tried and the underlying curl/wget stderr (e.g. curl: (6) Could not resolve host).
• Validates the downloaded file is non-empty before chmod, so a zero-byte response from a CDN/proxy does not silently overwrite the binary.
• Emits a clear error when the latest tag cannot be resolved (no internet / no git).
• Accepts BASHUNIT_INSTALL_DIR for testability and uses an absolute target path instead of cd-ing into src/.
• bashunit::io::download_to switches curl to -fsSL so HTTP errors (404/5xx) become real failures, and lets stderr through so the caller can surface it.

Files

• src/upgrade.sh — fixed control flow, error reporting, env-overridable install dir
• src/io.sh — curl -fsSL, stderr no longer swallowed, dependency miss message
• tests/unit/upgrade_test.sh — new unit tests covering all failure paths
• CHANGELOG.md — Unreleased entry

Test plan

• ./bashunit tests/unit/upgrade_test.sh — 6 passing (download fail, error message surfacing, empty file, missing tag, success path, already-on-latest)
• ./bashunit tests/ — full suite green
• make sa (ShellCheck)
• make lint (EditorConfig)
• Pre-commit hook (full suite + static analysis) passes