Download: add a 'resume' option

[fingolfin]

An interrupted download starts again from zero. For a data set of any size
that is the difference between a retry and an afternoon.

With resume := true and target given, the via curl method continues the
partial file (-C -), and a failure keeps that file so the next attempt can
carry on.

Methods that cannot resume decline rather than proceed, because they run
before via curl and would destroy the file it needs: wget's -c does not
resume when combined with -O — the output file is always created anew — and
SingleHTTPRequest has no range support. Measured before adding the decline:

via SingleHTTPRequest   file=fail     (deleted)
via wget                file=fail     (truncated by -O, then removed)
via curl                file=01234    (kept)

If no resuming method is available the download fails and the caller can
retry without resume.

The test server answers a Range request with the remainder in upper case, so
the test distinguishes a resumed download from a restarted one; removing the
-C - makes it fail with did not resume: abcdefghijklmnopqrst.

Based on #103, which introduced the "no target file after a failure"
guarantee that resume has to except.

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

CC @ThomasBreuer

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.38%. Comparing base (b7cd336) to head (a7c6643).

Additional details and impacted files

@@                      Coverage Diff                       @@
##           mh/download-target-cleanup     #105      +/-   ##
==============================================================
+ Coverage                       86.23%   86.38%   +0.14%     
==============================================================
  Files                              32       32              
  Lines                            1918     1939      +21     
==============================================================
+ Hits                             1654     1675      +21     
  Misses                            264      264              

Files with missing lines	Coverage Δ
lib/download.gi	93.16% <100.00%> (+0.67%)	⬆️

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[cdwensley]

@fingolfin Be sure to tell me when these four (or more?) PRs are ready to be merged and a release made.