Add a targetFile option for downloading straight to a file

[fingolfin]

DownloadURL builds the whole response body as a GAP string, so the largest
file it can fetch is bounded by memory.

With opt.target set to a filename the body goes straight to that file via
CURLOPT_WRITEDATA, and the result record has no result. A 200 MB download
costs 158 MB peak RSS instead of 334 MB; the written file's SHA256 matches.
On failure the file is removed.

The option is called targetFile, not target, on purpose: utils uses
target for its own Download option and forwards the whole record to
DownloadURL, so taking that name would break Download(url, rec(target := ...))
for anyone on an older utils. With a name utils does not forward, all four
version combinations work, and gap-packages/utils# makes utils pass
target on as targetFile to get the memory saving.

Tested against GAP 4.12.2, the declared minimum, and current GAP.

Two open questions:

• CURL_REQUEST goes from 8 arguments to 9. It is undocumented and
  CurlRequest fills in the new one, but direct callers break —
  tst/errors.tst asserted the arity. Keep it at 8 instead?
• Removing the target on failure forecloses CURLOPT_RESUME_FROM, which needs
  the partial file. I chose the safe default; resume would want an opt-in.

Written with Claude Opus 5 via Claude Code; reviewed by me.

[codecov]

Codecov Report

❌ Patch coverage is 90.69767% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.2%. Comparing base (6db0e56) to head (e1eec7d).

Files with missing lines	Patch %	Lines
src/curl.c	89.1%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #62     +/-   ##
========================================
- Coverage    95.3%   94.2%   -1.2%     
========================================
  Files           3       3             
  Lines         152     190     +38     
  Branches       15      22      +7     
========================================
+ Hits          145     179     +34     
- Misses          3       5      +2     
- Partials        4       6      +2     

Files with missing lines	Coverage Δ
gap/curl.gd	100.0% <ø> (ø)
gap/curl.gi	100.0% <100.0%> (ø)
src/curl.c	91.9% <89.1%> (-1.3%)	⬇️

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