fix(layout): reserve clear nozzle exits and recover blocked routing projections

[Alpha9463]

Summary

Stops automatic layouts from drawing a stream through equipment when a nozzle's exit is blocked. The router now recovers a reachable exit, default routing undoes the row alignment that seals one, and vertical reactors can discharge from a shell wall.

Related Issues

Closes #355
Refs: #547

Changes

• Recover a reachable nozzle exit by default: a blocked stand-off retreats along its outward ray to the nearest clear lane. A fully sealed nozzle still draws the fallback and reports route-crosses-unit.
• Add refine_exits to default routing: when a blocked-exit conflict remains, re-lay the sheet with the two units left unaligned and keep the result only if hard conflicts fall. At most three rounds; pinned units never move.
• Give the default and plain reactor bodies east and west outlet placements beside the floor outlet.
• On process equipment, skip a face whose stub crosses another unit, and take an alternate face only when it needs no more bends than the symbol's own.
• Close a Tk preview window after running its idle tasks, which stops Tk 9 on macOS from aborting the process.
• Pair path searches with streams by nozzle position in scripts/route_quality.py, since a sealed stream is drawn without a search.
• Regenerate docs/gallery/ for 01, 10 and 18 and tests/golden/18_fixed_bed_recycle.svg: in each, a reactor outlet moves from the floor to a shell wall.
• Rewrite docstrings and comments in the touched files to NumPy style.

Not included: side outlets on the jacketed and mixing reactor bodies, and exit repair on sheets placed equipment-first (free valve stations).

How to Test

1. Run the focused tests:

   python3 -m pytest -q tests/test_routing.py tests/test_faces.py tests/test_layout_refinement.py tests/test_show.py tests/test_golden.py

2. Run the complete suite and static checks:

   python3 -m pytest -q
   python3 -m ruff check .
   python3 -m ruff format --check tests
   python3 -m mypy pandid
   python3 -m pyright pandid

3. Measure the corpus with pins stripped and confirm no stream is listed as drawn by the fallback:

   python3 scripts/layout_quality.py --worst 10

Evidence

Measured on 46 drawings: the 21 shipped sheets as authored and with pins stripped, plus the four biodiesel variants in tests/_layout_cases.py.

	main at 17d32b3	This branch
Fallback routes	4	0
route-crosses-unit	8	0
Hard conflicts	18	0
Crossings	257	260
Bends	1117	1118

• Local suite: 11473 passed, 188 skipped. Ruff, mypy and git diff --check are clean; pyright reports 0 errors and the existing __all__ warning.
• Pins-stripped 21_alumina_refinery gains four crossings (65 → 69) in exchange for two fallback routes and four route-crosses-unit findings.
• Pinned biodiesel gains one run-off-elevation warning: R-101's east outlet sits 2.3 px off S-101's feed.
• scripts/layout_compare.py has no review file for this change, so its release gate is not evaluated here.
• Before/after drawings: biodiesel (pins stripped), 21_alumina_refinery (pins stripped), 01_ammonia_loop, 10_ethanol_pfd, 18_fixed_bed_recycle.

Checklist

• Base branch is main and the branch is mergeable
• Commit subjects follow Conventional Commits; tracked work links its issue
• Self-reviewed the full diff on the Files changed tab
• Relevant local checks pass; all CI jobs pass before merge
• Documentation and examples updated if behaviour changed
• Before/after drawings included if rendered output changed
• Blocking review feedback resolved, if review was requested