[Parity]: [P3] Old-save 4-wide walkway runs cannot be continued after E1b widening (watch-only)

Imported from the engineering backlog (_bmad-output/implementation-artifacts/backlog.md, story e1b-widths).
Priority P3 (do-first order) | Severity: low | Status: open | Epic: SimTower parity | Type: review-deferral | Recorded: 2026-07-07

Divergence from the 1994 original

Ready, arguably-correct, watch-only. After E1b widened stairs/escalator 4→8, an OLD-save 4-wide walkway run can't be continued with a new 8-wide flight in the same column: the stacked-flight landing-share exemption (Tower.ts:541-548) requires t.width === f.width, so the mixed-width overlap is rejected as "shafts cannot overlap." Only affects extending a pre-E1b stair/escalator run; mixed-width shafts arguably shouldn't merge, so this may be correct-as-is. If it reads as a bug, allow the landing-share when the columns align regardless of width, or bulldoze+rebuild the run. (Edge Case Hunter, E1b review, Finding 2.)

Start with the stacked-flight landing-share check in Tower.ts:541-548 and the e1b-widths backlog story. Examine the old-save 4-wide run followed by an 8-wide flight, then determine whether the mixed-width overlap should remain rejected or be treated as a continuation. Done means the behavior has a decided, documented outcome and the issue is no longer watch-only.