Avoid unnecessary sudo in install, use safe test cleanup, and relax perf threshold

[tw93]

Motivation

• Prevent installing as root into writable temporary prefixes by ensuring needs_sudo checks the parent directory when INSTALL_DIR does not exist.
• Make test cleanup safer by using existing removal utilities instead of raw rm -rf.
• Reduce fragility of the bytes_to_human performance test by allowing a configurable threshold for timing variability.

Description

• Update needs_sudo in install.sh to return writable checks for INSTALL_DIR when it exists and otherwise check the parent directory.
• Source lib/core/file_ops.sh in scripts/test.sh and replace the raw rm -rf /tmp/mole-test with safe_remove "/tmp/mole-test" true.
• Change tests/performance.bats to use a configurable limit MOLE_PERF_BYTES_TO_HUMAN_LIMIT_MS with a default of 4000 ms instead of a hard-coded 2000 ms.

Testing

• No automated tests were executed as part of this change.

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Codex Task