[Parity][P3]: a pre-v1.14.0 stairless export with a skip-floor shaft cannot be recognized

Accepted limitation from PR #742, recorded so it is a decision rather than a surprise.

The shape

A .TDT that is ALL of:

• exported by Verticopolis 2.9.0 or earlier (serviced-floor payload layout), AND
• exported before v1.14.0 (so the file stops after the stairs table instead of carrying the 0xFF routing region), AND
• stairless (its 64 stair records are empty), AND
• carrying a shaft that skips floors.

Why it is not fixed

The importer takes the legacy layout only when the file corroborates it, by placing a structure whose old-writer position is known exactly where that walk's table would end. This shape has no such structure: no stairs table to anchor on, no routing region.

Anchoring on the file's END was tried (c9f28e3) and withdrawn one round later. A CURRENT-layout file truncated at exactly the shorter walk's end plus finance, parking and stairs is byte-identical to this shape whenever both regions are zeros, which they are for a stairless tower with no connected stalls. Nothing can separate them, so an EOF anchor cannot recover the old file without silently mis-reading the damaged one, and a silent wrong answer is the worse failure. Both directions are pinned by tests in tdtImport.integration.test.ts.

What a player sees

The import falls back to synthesized transports WITH a warning, which is the pre-#742 behavior for these files. Nothing is silently wrong; the tower's own shafts are not recovered.

If it ever matters

The population is tiny: exports from before v1.14.0 are the ones that page-faulted the 1994 game on load (backlog tdt-trailing-structure-layout), so they never worked for their only purpose. Recovering them would need a positive marker inside the file, e.g. stamping our own exports with a format-generation byte, which is worth doing for its own sake if we ever revise the writer: it would replace all of this inference with a fact.

Story id: tdt-legacy-pre-tail-import

Start with tdtImport.integration.test.ts, which pins both the indistinguishable legacy and truncated current-layout cases, and review the importer path that chooses the legacy layout. This issue specifies no implementation; completion would require an agreed positive format marker and corresponding importer and test changes.