v0.2.1: sharper tool docs for the C tools, and a working geolocation

0.2.1: sharper tool docs for the tools that were still C, and a working geolocation

Glama re-scored the 0.2.0 tool set: outline went 2.9 -> 5.0, but
heap_snapshot and trace_stop landed at 2.9 (the server score weights the
minimum tool at 40%), so their descriptions now say what they write, where,
and what they overwrite. upload_files, perf_metrics and emulate were
rewritten the same way.

• emulate(geolocation) grants the geolocation permission for that http(s)
  origin before setting the position; without the grant the page could not
  read it. The grant is never context-wide, so an attached real browser
  (--cdp/--user-data-dir) does not silently hand every site a position.
• perf_metrics now includes the Nodes counter its description promised
• heap_snapshot/trace_stop/upload_files document --file-root (and that it
  must be explicit when the server runs from / or $HOME)
• tests: 240 (Nodes counter, geolocation readable from a localhost page,
  trace_stop without trace_start is an error)

Reviewed by oc-review over two passes.