v1.17.1

Fixed

• A sub-block span now keeps a lead instead of being placed once
  (#407). This is the cause of v1.16.0's rolled-back
  regression, fixed; the flag stays off by default.

  The span bounded the dispatch a handle got at establishment and was never extended. The
  whole-block frontier cannot move again until the reader crosses a block — 64 reads at the
  default 8 MiB block and a 128 KiB read — so a handle that went on streaming faulted through
  everything the one span did not cover. Measured externally at median 0.798 throughput with
  bytes identical: the entire cost was faults and wall.

  The span now holds the frontier earned bytes ahead of the reader and is extended as the reader
  consumes it, including within a block, which is where the detector previously returned
  early for every read that did not cross a boundary. That is the evidence-gate rule expressed in
  bytes rather than blocks: a handle that keeps reading contiguously earns a deepening lead and
  converges on what the whole-block path would have given it, while one that stops has committed
  only what it had read.

  Dispatch now carries an absolute span range (SpanOff, SpanLen) rather than a length the
  FUSE layer placed at the read's end. An extension begins where the previous span ended, which
  only the detector knows — the original split was elegant and could not express the fix.

  The cost is some of the byte saving, which is the right trade and is stated rather than
  elided: a 64-field walk fixture goes from 12.1× to 8.6× less dispatch, and the end-to-end
  figure from 2.03× to 1.87× fewer bytes.

Added

• A deterministic guard for the axis the previous witness missed. The earlier
  scatter-then-stream test asserted bytes and span count; bytes were identical and the whole cost
  was faults and wall, so it passed and the regression shipped. Fault counters are too noisy in a
  fake-server fixture to assert on, but dispatch coverage in byte space is deterministic — it
  is a function of the decisions, not of timing — so that is what the new test asserts.

  It sweeps the stream's phase relative to the block grid, which is not decoration. At a
  block-aligned start the handle establishes 64 reads in with a near-floor span and the gap does
  not appear at all; a first version of the probe used an aligned base and reported the two arms
  identical, which reads exactly like "no regression". The gap is largest when the handle
  establishes early, having earned little:

  stream phase	established at read	uncovered, whole-block	uncovered, span placed once
  +0 MiB	64	65	65 (invisible)
  +2 MiB	48	49	73
  +6 MiB	16	17	105
  +7 MiB	72	73	73 (invisible)

  With the lead maintained, every phase matches the whole-block arm. Three further properties are
  pinned because a revert sweep found each one ungated: dispatches stay a small fraction of reads
  and never re-cover ground; a walk read backward gets as many spans as the same walk read
  forward (the span frontier is absolute, so failing to reset it at a run boundary silently
  starves every subsequent run — 35 spans became 7); and the FUSE layer passes the dispatch's own
  offset, checked by an AST walk because no behavioural test in that package can see it.

  One honest note recorded in the test: the frontier's early-return is defensive and not
  exercised by any measured shape. Removing it left the dispatch count at 48 of 512, unchanged,
  because earned grows twice as fast as the reader advances. The dispatch-count assertion guards
  the floor check, which is load-bearing, rather than the frontier.