Resume a dropped mesh download where it stopped, without counting it as a failed attempt

[Robbie1977]

A mesh body that broke off part-way was retried from the top, and after four such drops handed to the server fallback - which fetches the same URL over a worse channel. For a user on a weak or long path (the mobile/continent skew in the GA breakdown) that is the wrong response to an ordinary interruption.

Client VFBv2.4.5.10:

• fetchWithRetry counts two kinds of failure apart. A request that got nowhere (refused, HTTP error, body dead before a new byte) is a strike; four strikes end the call as before. A body that dropped after making progress is not a strike: the download resumes from the last byte parsed with Range: bytes=N-, and the strike count resets. RESUMES (8, window.VFB_FETCH_RESUMES) bounds how many interruptions one call rides out. An HTTP error on a resume keeps the offset.
• createObjConsumer() keeps the parser and decoder across requests, so a resumed body is more chunks to the same parse. A 206 is spliced in only if it is the same version of the file (strong ETag where both responses show one, else Last-Modified) and its Content-Range starts exactly at the parse position (or, where that header is hidden, its Content-Length pins the start). A 200 restarts the parse; any other 206 is refused. Both silent-corruption cases - a partial body parsed as a whole file, a whole file fed into a half-fed parser - are tested.
• No If-Range: it made the browser preflight with OPTIONS, which the data hosts answer 405, so no resume ever left the page. The version check above replaces it.
• attempts is now every request the load made; resumes is new alongside it. 16 new tests (61 total).

This repo: client pin bumped; geomfail/georecover carry :rN when any request was a resume (absent otherwise, so existing names are unchanged).

Verified on v2-dev against real data hosts: a 9.6 MB mesh cut at 200 KB on cayenne resumed on buttermilk with a 206 at the right offset and matching ETag, reported georecover:obj:jrchjrcj:a2:r1, and the resulting mesh has exactly the file's vertex and face counts. Needs Access-Control-Expose-Headers: ETag, Content-Range, Accept-Ranges on the data hosts for the strong validator (now set in HAProxy); falls back to Last-Modified and Content-Length without it.