[Parity]: [P3] TDT quirk A/B round-trip faithfulness (needs own spec + party)

Imported from the engineering backlog (_bmad-output/implementation-artifacts/backlog.md, story tdt-quirkAB-faithfulness).
Priority P3 (do-first order) | Severity: low | Status: idea | Epic: SimTower parity | Type: feature-request | Recorded: 2026-07-13

Divergence from the 1994 original

Gated: needs its own spec + party. Owner asked (overriding the earlier party ruling that A/B are canonicalizations, not bugs) to make the round-trip FAITHFUL for the two TDT canonicalizations documented in docs/canon/tdt-format.md §14: (A) a plain floor on a sky-lobby story (15/30/45...) imports back as a lobby; (B) a vacant never-simulated built shop imports as occupied+subtype. Both are lossy at the TDT seam because the format can't represent the pre-canonical state. The RIGHT fix is engine-side, NOT touching parseTDT (Cloud's standing ruling): stop the engine from PRODUCING states the format can't hold, i.e. (A) a placement rule so a sky-lobby story can't carry bare floor tiles without a lobby, (B) assign a retail subtype at placement so a built shop is never subtype-less. This is a gameplay-adjacent engine change -> full gds-code-review + a Samus feel check; write a gdd-/spec first. Deferred from the 2026-07-13 tdt-fixes session (party: do not rush engine placement changes at a session tail).

Start with docs/canon/tdt-format.md §14 and the imported story in _bmad-output/implementation-artifacts/backlog.md to understand both lossy canonicalizations. Identify the engine placement entry points involved, then prepare the requested gdd-/spec before implementation. Done requires an agreed spec, engine-side prevention of unrepresentable states, full gds-code-review, and a Samus feel check.