fix: _vlog returns 0 when verbose=false (regression in install rc)

[Chemaclass]

Summary

Senior-QA acceptance run against 0.4.1 (with the README quick-start layout) caught a deeper bug than the clean/doctor self-exclusion fix:

`bashdep::_vlog` ended with the chain `bashdep::is_verbose && bashdep::_log "$@"`. When verbose is off (the default), `is_verbose` returns 1, the chain short-circuits with rc=1, and that becomes the function's exit code. `_vlog` is the last statement of `bashdep::download_url` (` lockfile: $lock_file` line, added with the verbose-mode commit), so `download_url` returned 1 on every successful download.

`bashdep::install` then counted each successful install as a failure:

```bash
bashdep::download_url ... || failures=$((failures + 1))
```

So `install` returned the number of "failures" (= number of installed deps) and `install_from` propagated it. Real-world impact:

```bash
source lib/bashdep
bashdep::install_from .bashdep || exit $? # exits 1 on a perfectly successful install
```

Why unit tests missed it

No test asserted `download_url` returns 0 on a real successful download:

• `test_bashdep_install_returns_zero_when_all_succeed` mocks both `setup_directory` and `download_url` to `return 0`, bypassing the real path.
• Snapshot tests check stdout, not the function's rc.
• `test_bashdep_download_url_writes_lockfile_entry` checks file state, not rc.

The bug only fired when verbose was off (default) AND a real download path ran end-to-end. Mocked download_url tests never exercised `_vlog`.

Fix

```bash
function bashdep::_vlog() {
if bashdep::is_verbose; then bashdep::_log "$@"; fi
return 0
}
```

Always return 0 — "not verbose" is a normal state, not a failure.

New tests (7)

• `download_url` returns 0 on success (default path)
• `download_url` returns 0 with verbose=false
• `download_url` returns 0 with verbose=true
• `install` returns 0 after a real download (not mocked)
• `install_from` returns 0 after a real download
• `_vlog` returns 0 with verbose=false
• `_vlog` returns 0 with verbose=true

Test plan

• `make test` (115 tests, 170 assertions)
• `make sa`
• `make lint`
• Reproduced the bug with `bashdep::install_from .bashdep || exit $?` against the published 0.4.1; confirmed fixed on this branch.
• Cut 0.4.2 after merge