fetch-pack: parallelize no-ref-delta URI packs

[ttaylorr-oai]

Summary

This is the reduced stack for no-ref-delta URI-pack indexing.

• Carries the four commits from
  tb/upstream/no-ref-delta
  first.
• Keeps Friel's work as three commits: index-pack --no-ref-delta, v2
  fetch no-ref-delta support, and the parallel fetch path. Friel's author
  and Signed-off-by identity are preserved.
• Uses one v2 fetch no-ref-delta contract for the inline pack and every
  URI pack. upload-pack passes --no-ref-delta to the inline
  pack-objects; the administrator must create or verify configured URI
  packs with the same property. The client requests the promise only with
  packfile URIs and fetch.packfileUriJobs > 1, verifies the inline pack
  and every URI pack with index-pack --no-ref-delta, and gives URI
  indexers --threads=1 before running them concurrently.
• Uses the existing http-fetch --packfile helper. Expected keep files are
  created up front and entered in the existing pack-lock list before helpers
  start; pre-existing keeps are left alone. A small poll/refill loop bounds
  concurrent helpers. Direct download-to-index-pack streaming remains out
  of scope.

The final feature commit is 263 insertions / 11 deletions, down from
519 / 5 in the first reroll. The FIFO, background-job, polling-shell,
lying-server, duplicate-hash, one-job, invalid-config, and standalone
tempfile-cleanup tests were removed. The remaining coverage is
compositional: t5300 checks REF_DELTA rejection, and t5702 checks the
unified fetch negotiation, server-side pack-objects flag, inline verifier,
and three URI indexers with two jobs.

Testing

• make -j8
• (cd t && ./t5300-pack-object.sh --run=1-40) (selected tests pass)
• (cd t && ./t5332-multi-pack-reuse.sh) (15/15)
• (cd t && ./t5516-fetch-push.sh) (129/129)
• (cd t && ./t5702-protocol-v2.sh) (88/88)