v1.17.0

Changed

• --readahead-subblock-dispatch is off by default again, reverting v1.16.0
  (#407). It costs wall clock on an
  index-then-stream reader, which v1.16.0's evidence did not cover.

  shape	bytes	wall, median B/D	uncovered misses
  GRIB .idx sweep	3.02× → 1.39×	1.005 (11 kept reps)	69 every arm
  HEMCO time-slice	1.79× → 1.48×	0.974 (5 kept reps)	331–333 every arm
  index-then-stream	identical	0.798 (5 kept reps)	62 → 81

  The last row was below both of its brackets in 8 of 8 reps. The mechanism is the first
  dispatch at establishment — one ~5.5 MB span in place of three whole blocks (24 MiB) — after
  which a streaming reader faults through the uncovered remainder until the next block crossing;
  the window schedule after that is identical in both arms.

  The trade is bytes on a walk that stops against wall on a walk that continues, and which
  of those a mount is cannot be known at establishment. That makes it an opt-in, not a default.
  The 13-shape sweep was otherwise clean and the default did wire through, so nothing else is
  affected either way; webdataset_stream was neutral over 8 kept reps, confirming the default
  cost nothing where it did nothing.

  Turning it back on wants a span that still leads the reader rather than only shrinking the
  first dispatch — not a re-run of the same cells.

Fixed

• The witness built for this exact risk asserted the wrong axis. internal/fuse's
  scatter-then-stream test was written before the arm shipped, specifically to catch a handle that
  scatters and then streams, and it asserts bytes and span count. Bytes are identical on that
  shape; the entire cost is in faults and wall. It passed, the default went on, and the regression
  shipped.

  And it cannot be fixed by adding an assertion there. The fault counts are now logged, and
  they are too noisy in that fixture to gate on: across three identical runs the arm's extra
  faults came out +5/+1/+0 on the short stream and +4/+4/+11 on the long one, so even a contrast
  between the two already fails. UncoveredMiss fires when a demand read finds its chunk neither
  cached nor in flight, so it races the prefetch goroutine, and a fake server has no latency for
  that race to resolve against. The external cell needed 1 GiB streams and 8 bracketed reps to see
  62 → 81 cleanly.

  So the figures are logged with a comment that says plainly a green run of that test does not
  clear this arm — the wall and fault axes are external-only. Writing a flaky assertion there
  would have been worse than writing none.