The server clamped window_n but trusted window_center — the letterbox
bug's impossible (12276, 3021) was derived at clamped-garbage latitude
and CACHED under its raw key. normalize_window_center mirrors the
forward map exactly (column rem_euclid against the body's district
circumference, row clamped to +/-half-meridian; constants from
scale::DISTRICT_M, no new magic numbers; no-radius test bodies
identity). Applied in serve_district_window BEFORE the window-cache
key and the coalescing key, so an insane request and its sane twin
share one entry — proven end-to-end with the literal bug values on a
500km fixture body resolving to canonical (4, 383), byte-identical.
Echo contract: DistrictWindowLayer.center now carries the NORMALIZED
value. Reconciled by design with the client half in this same branch —
the client canonicalizes before sending (bit-identical math, posmod ==
rem_euclid verified), so request and echo agree from the first
round-trip; server normalization is defense-in-depth for buggy or
modified clients.
6 new tests incl. the twin-cache proof and the echo pin; atlas::
570/570, lib 1784/1784, timing-sensitive tests 3x stable.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
jpmschweitzerandClaude Fable 5
4a65bb210d