Split coil-clearance PR into reviewable units

[krystophny]

PR #298 mixes engineering logic with test-performance changes. Split it before merge so each review has one risk profile.

Proposed sequence:

1. Add coil_clearance.py and analytic geometry tests.
2. Wire the clearance constraint into the port optimizer.
3. Add plot/artifact generation in a separate PR if needed.
4. Parallelize the chartmap wall-loss test separately.

Acceptance:

• Each PR states its risk tier and correctness contract.
• The OpenMP/test-performance change has no engineering behavior change.
• The engineering PRs can be reviewed without reading unrelated test scheduling changes.