A second download source, for when the first one refuses

Cloudflare rate-limits by volume, and the block it handed out while this was
being built lasted over twenty minutes — too fragile to be the only thing behind
a button. So the download phase tries its sources in order: the anycast endpoint
first, then a plain 100 MB file on Linode's Seattle mirror, which has no opinion
about how much has been pulled today.

Only a refusal moves to the next one; anything else would fail the same way
twice. Every result names the source it used, and the page says "from
linode-seattle" rather than "via SEA" when it was the fallback — that mirror is
in one fixed place, so somewhere else it reads low, and a figure that quietly
came from a different continent would be a lie.

Upload stays on Cloudflare, which has never been the one to refuse.

Also: the too-short-window guard was refusing to divide 512 MiB by ten
milliseconds on a link fast enough to hit the runaway budget before the first
tick. It now guards against dividing by zero and nothing more.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/src/api.ts

@@ -182,7 +182,7 @@ export interface NetReport { at: string; host?: string; ifaces: Iface[]; gateway
 export interface Outside { at: string; ip?: string; colo?: string; country?: string; error?: string }
 export interface SpeedResult {
   at: string; down_mbps: number; up_mbps: number; latency_ms: number; jitter_ms: number
-  down_bytes: number; up_bytes: number; took_sec: number; colo?: string; ip?: string; note?: string; error?: string
+  down_bytes: number; up_bytes: number; took_sec: number; colo?: string; ip?: string; source?: string; note?: string; error?: string
 }
 // A test in progress, or the last one to have finished. `done` is 0–1 across the whole test
 // and `mbps` is the live figure for the phase under way.