[Feature]: [P3] Post-reflow W1/W2 wave feel on migrated wide towers

Imported from the engineering backlog (_bmad-output/implementation-artifacts/backlog.md, story w1w2-post-migration-wave).
Priority P3 (do-first order) | Severity: low | Status: open | Epic: SimTower parity | Type: review-deferral | Recorded: 2026-07-07

Problem / motivation

Ready, feel, not correctness. After a v1→v2 reflow re-lays a returning player's wide tower on the 375 lot, a cohort of offices can cross the 79-tile (W1) / 11–21-tile (W2) lines and begin eroding together, a batched-but-large notice a day or two after load on a tower that "was fine." The erosion is gradual and telegraphed (one batched toast, 2-day notice, recoverable), so it's not a contract violation, but it can read as the game breaking a working tower. Consider a short post-migration grace (suppress W1/W2 erosion for the first in-game day after a v1 load) or a one-time explanatory toast ("Canon spacing rules now apply, check flagged units"). At minimum, confirm the golden towerone_6 fixture doesn't dump a mass notice on first load. (Game Designer, E2 review, Finding 3.)

Start with the w1w2-post-migration-wave story in _bmad-output/implementation-artifacts/backlog.md and inspect the golden towerone_6 fixture and its first-load behavior. Determine whether the expected outcome is a one-day post-migration grace or an explanatory toast; the work is done when the fixture no longer produces an unexpected mass notice and the chosen behavior is verified.