Publish a pull request under pr/, not beside the branches (#307)

[adrpo]

A pull request run was named pr-<N>, so its results landed in
branches/pr-16370 next to master, v1.27, cpp and the 65 other
directories there, and its reports in branches/history/pr-16370. One directory
per pull request tested would bury the branches.

The name is pr/<N>, which is a directory of its own:

branches/pr/16370/                          the per-library results
branches/history/pr/16370/                  the report and the summary

The one name that keeps its directory

Every other job is named after a git branch and takes only the last part of the
name - maintenance/v1.27 is tested, stored and published as v1.27. That rule
was a split("/")[-1] in each of report.py, all-reports.py, all-plots.py,
single-model.py and pr-report.py; it is now one place:

prBranchRe = re.compile(r"^pr/[0-9]+$")

def resultTable(branch):
  """The results of a job named after this branch: its table and its directory."""
  return branch if prBranchRe.match(branch) else branch.split("/")[-1]

report.py --branches='master pr/<N>' works as a side effect, which it would not
have before: the name would have been cut down to the number.

drop-pr-tables.py matches pr[-/]<N>, so the tables of the runs made before
this are dropped by the same rule.

The run that is already there

PR #16370 was tested before this - pr-16370, 19661 models - and stays exactly
where it is. Nothing here moves it and nothing needs to: this is for the runs
from here on. drop-pr-tables.py matches pr[-/]<N>, so it goes the same way as
any other once the pull request is merged or closed.

Tested

Against a sqlite database whose pull request table is pr/1234:

./pr-report.py 1234	writes history/pr/1234/, and the model links point at branches/pr/1234/... while the baseline ones point at branches/master/...
./report.py --branches='master pr/1234'	the pull request column links to pr/1234/...
./drop-pr-tables.py	lists pr/1234 and pr-16357 together, drops the merged one
the sanity check the CI runs	test.py --branch=stable configs/sanityCheck.json, then report.py, then the grep from .github/workflows/test.yml: passes

---

Generated by Claude Code.