fix(download-proxy): Report telemetry per download, not per range

[kotevcode]

Summary

GA4 showed more `download_completed` events than `download_vpr` clicks on ad-wave days. Two causes, one of which is fixable in the Worker:

1. Per-request reporting. Download managers (IDM etc.) fetch one installer as N concurrent byte ranges, and browsers resume a paused download with a new `Range` request. Each request emitted its own `download_started` / `download_completed`. Over the last 14 days, `partial=1` was 142 Windows completions from just 50 users (~3×).
2. Ad blockers. `download_vpr` is a browser-side GTM event; the proxy events are server-side. 38% of completions arrived with no GA ids (no `_ga` cookie → GA never ran in that browser), so those visitors can't have fired the click event at all. Not fixable, but now visible.

Changes

• `segmentRole(status, contentRange)`: a 200 is first+final; a 206 is first when its `Content-Range` starts at byte 0 and final when it ends at `total-1`. Unparseable or `*`-total ranges fall back to first+final so a misbehaving origin never drops a download.
• `download_started` is emitted only for the first segment; `download_completed` / `download_aborted` only for the final one. Middle segments are logged to the console as `download_segment_completed/aborted` and never sent to GA. `partial=1` stays on the reported events so download-manager users remain identifiable.
• Every GA4 event now carries `attributed: 1|0` (previously console-only), separating visitors whose browser passed GA ids from those where GA was blocked.
• README telemetry section rewritten; CHANGELOG entry under Unreleased → Changed.

Result: an 8-range IDM download yields 1 started + 1 completed instead of 8 + 8; a browser pause/resume yields 1 started, 1 aborted, 1 completed.

Known edge: a Safari-style `bytes=0-1` probe still emits one extra `download_started` (no end event).

After merge

• `pnpm --filter @antseed/download-proxy deploy`
• GA4: register `attributed` as an event-scoped custom dimension; consider moving the key event (PMax target) from `download_completed` to `download_started`, or keep `completed` now that it is 1:1 with downloads.
• Funnel: `download_vpr → download_started → download_completed` on unique users; read `download_vpr` for page/section breakdowns only.

Testing

• `pnpm --filter @antseed/download-proxy typecheck` — clean
• `pnpm --filter @antseed/download-proxy test` — 25 passed (10 new: segmented download, browser resume, fallbacks, middle-segment naming, `deliverEvent` payloads for attributed/unattributed/unconfigured)