PKCE exchangeFailed on first sign-in: instrument shipped, population gone, unresolved between two lanes

Carried out of TERRA-18, which shipped its other half and closed. The defect: at 07:12:25 on 2026-08-14 a real first-time client sign-in logged signInCompleted level=error failureCode=exchangeFailed, detail naming a PKCE code verifier not found in storage. db-terra-cc read it as browser-side and benign, coder-terra-cc did not, and that disagreement is still unresolved - it is why this is a row and not a note. NOT FIXED by 6f9a0d0: no cookie-host cause was found and none was fixed. What 6f9a0d0 shipped is a diagnostic that makes the leading hypothesis MEASURABLE and nothing more - servedHost is now classified on the signInInitiated row, so a code_verifier written by createBrowserClient on a non-canonical origin and read back on the canonical one would show up as such. WHY IT CANNOT BE SETTLED THIS WEEK, and this is the part not to lose: TERRA-20's second front door was open on 2026-08-14 and closed on 2026-08-16, so the population that could exhibit the defect no longer arrives at all. The instrument is live and the population is gone. This shares TERRA-43's organic-traffic horizon of 2026-08-23T12:00Z - at that point either a servedHost value has attributed the failure, or this is UNRUNNABLE-ON-THIS-DEPLOY and closes with that stated, not with a verdict. DO NOT close it as fixed on the strength of the front door having been closed: that would credit a fix to a change that was made for another reason and never tested against this failure.