fix(routing): render true direction:RL serpentine multi-line folds as a clean U

[pinin4fjords]

Summary

A true boustrophedon serpentine (top row left-to-right, fold straight down on the right edge, return row right-to-left via %%metro direction: RL) carrying a 2+-line bundle could not render its fold corner: every fold-corner port combination either aborted in assert_render_curve_invariants (bundle flip + non-concentric descend→turn corner) or wrapped back through a section interior. This is the common shape for folding a long nf-core pipeline map that carries a multi-line bundle (surfaced by nf-core/rnadnavar#83).

The fold now renders as a clean U: a straight vertical descent down the right edge, with the bundle staying on the outside the whole way round (top, right, then bottom) and no crossover.

Fix

• routing/reversal.py — a horizontal-flow (LR/RL) section's BOTTOM exit feeding a horizontal-flow TOP entry now marks the receiver (and its row successors) reversed, just like the existing TB-feeder case. The descend→turn corner reverses bundle order, so the receiver's trunk order continues the fold rather than resetting to declaration order — the bundle rides the outside of the U with no crossover.
• routing/perp.py — the intra-section drop departs on the same per-line X the up-and-over feeder lands on (the entry port's own offset), so the section boundary stays straight (no S-jog).
• layout/phases/ports.py — the RL drop target's trunk shifts so its perpendicular entry port aligns under the exit column, giving a straight vertical descent. (A fully-aligned drop into the first station is not possible for a horizontal-flow target — the trunk is horizontal so the drop must turn, and station-as-elbow forbids the port sharing the station's X — so the entry port is the alignment reference.)

Regression infra

• examples/topologies/serpentine_rl_bundle.mmd fixture + Serpentine Layout gallery entry (so the CI render-diff tracks it).
• tests/test_serpentine_rl_bundle.py — asserts the fold renders without abort, the descend→turn corner is concentric, and bundle order is preserved.

Refactor (second commit)

Unifies the two near-duplicate drop-target traversals onto a shared _perp_drop_targets + lanes_run_along_x/y filters (migrating off raw direction-string tuples; TB-branch ratchet 23→22), and extracts the shared trunk-shift loop into _shift_section_trunk.

Test plan

• Full suite passes (25381 passed); new invariant test included
• ruff check + ruff format clean on whole repo; mypy clean on changed files
• guard-golden baseline regenerated (additive); seam-orientation residuals documented; TB-branch ratchet lowered to match the AxisFrame migration
• Visual review of the CI render preview (authoritative cross-gallery diff)

🤖 Generated with Claude Code

[github-actions]

Render preview is ready for review:
🖼️ https://seqeralabs.github.io/nf-metro/_pr/1202/

This preview shows only the renders that changed compared to main.