[Parity]: [P3] W3 lobby distance anchors only on ground floor (deep basements; metro-anchor idea)

Imported from the engineering backlog (_bmad-output/implementation-artifacts/backlog.md, story w3-basement-depth).
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, likely by-design, watch-only. W3 (nearestLobbyFloorDistance) anchors only on ground (floor 1), so a deep-basement commercial venue (B3 = floor −2 and below) sits >2 floors from a lobby and takes the permanent ×0.5 traffic penalty, and, because a lobby can never be placed in a basement (isLobbyFloor = floor 1 or ×15 only), it can never be restored, unlike an above-ground shop a sky lobby can rescue. Matches canon (deep-basement retail is genuinely far from the concourse) so shipping as-is, but if it reads as unfair, consider treating an operational metro floor as a W3 anchor (the canon underground entrance draws visitors), or clamp basement distance to the ground. (Edge Case Hunter, E2 review, Finding 3.)

Start by tracing W3 (nearestLobbyFloorDistance) and the isLobbyFloor rule described in the issue, using the backlog entry w3-basement-depth as context. Compare deep-basement behavior with the stated parity expectations; done means deciding whether metro floors should anchor W3 or basement distance should be clamped, then validating the chosen behavior.