fix(runner): skip inactive parallel spinners

[Chemaclass]

Background

Closes #1368.

Captured parallel runs keep their output pipes open for roughly one extra second because an inactive spinner starts a sleep child. Start the spinner only when stdout is a terminal, progress is enabled, and output is human-readable.

Changes

• Skip spinner creation and cleanup when no animation is visible.
• Return immediately from the spinner when it is disabled.
• Add a deterministic regression for piped output, machine output, and --no-progress. It detects spinner creation without an elapsed-time threshold.

Validation

make sa, make lint, the full sequential suite on macOS Bash 3.2 and Linux Bash 5.3, macOS make test/parallel and make test/strict, and the full built-artifact suite pass. Parallel checks use four jobs; macOS runs use the directory argument to avoid the existing Bash 3.2 process-substitution descriptor limit.

The full source suite passes on GNU Bash 3.00.0 in sequential, parallel, simple, simple-parallel, and strict modes, using the same shell selection as CI. The built artifact also passes 138 focused provider, reporting, output, and fork tests with Bash 3.0 first in PATH. A PTY check confirms ordinary terminal progress still starts the spinner and suppressed progress does not.

On macOS arm64 and Bash 3.2.57, 100 passing tests with --parallel --simple and both output streams captured improved from 1,257 ms to 439 ms, a 65% reduction. Redirecting output to /dev/null was effectively unchanged (397 ms to 391 ms). These are medians of five alternating baseline and branch runs after one warm-up, using the same external harness and fixture against main at 860c797a48e34be20b019a70f58c0e06d15c8d33. Timings stay out of the test suite.

Checklist

• I updated the CHANGELOG.md to reflect the new feature or fix.
• Documentation does not need changes; the CLI and public API are unchanged.