Records the after_step: driver seam as a proposed ADR-0008 amendment

[johnnyt]

Bead sp-sli (campaign-SF039 lane P, plan key pr1). Records the
after_step: driver seam as a proposed amendment at the end of
docs/adr/0008-durable-subchart-child-runs.md, per the operator's
campaign-SF039 ruling RQ-SF039-13.

What the amendment says

• S1 Driver.new/3 takes after_step:, a 3-arity
  (run_id, machine_state, effects) function, stored on the struct and
  nil by default. The third argument is the step's whole effect list, as
  Runs' persist tail was handed it, not the executable subset the
  driver's own executor sees; the seam that carries it out of Runs is
  sp-c48's, with a widened public Runs return ruled out.
• S2 It fires after every Runs entry point the driver calls -
  Runs.create/4 and the private step/5's Runs.step/5, reached from
  the answer loop and from both late-answer doors - with the id of the run
  that was stepped. That covers the parent's step on the answer path,
  including Runs.fail/4's driver: path, and it reaches nested drives
  for free because a child driver and a resolved parent driver are both
  built as %{driver | machine: ...}. Runs.cascade_cancel/3 steps
  nothing and fires nothing.
• S3 After the persist, in the order the steps happened, outside the
  exclusion of the run it reports. The section states the scope of that
  guarantee explicitly: decision 3's single child is created from inside
  the parent's step, so a child's callback can fire while the parent's
  exclusion is held; what is promised is that a run's callback never runs
  inside that run's own exclusion.
• S4 The return is ignored; a raise propagates to the caller, and the
  cost (a part-taken drive, recovered by the same at-least-once re-drive
  as every other window in the record) is stated.
• S5 Not a telemetry event - ADR-0009 stays untouched, and the two
  seams' opposite failure semantics are why both exist - and not a way to
  alter the step.

A worked example (the outside fail of a child answering its parent, the
host appending one row per parent step) and a "what this does not decide"
paragraph close the section.

Status

Proposed, with a status line so there is a line to flip. sp-c48
implements it; sp-nhl flips this section once that has landed. No lib/
change ships here - the same posture the sp-n8g amendment above records
for itself.

Provenance and checks

• Appended at the END of the file, after the sp-y7n note it extends.
  git diff origin/main -- docs/adr/ is 188 insertions, 0 deletions.
• Every code cite names a function-head anchor and the SHA it was read at
  (4e3e2c8); all were re-verified by anchor immediately before this
  request was opened.
• Gate: not run, and none applies. The diff touches one Markdown file
  under docs/adr/ and no Elixir code, which is the CLAUDE.md authority
  table's "a change touching no Elixir code has no gate to run and may
  commit on review of the diff alone". CI runs the full gate on this
  branch regardless.
• No changelog fragment: changelog.d/README.md lists documentation
  and ADRs on its "do not write a fragment" side.
• Terminology firewall scan run over the outbound diff, bead text and this
  body, with a constructed positive control: clean, control fired.

[johnnyt]

Direction review pass 1 (head 16d982e): QUALIFIED, 1 finding, 0 blocking.

Finding (RECORD-PROSE, NON-BLOCKING): the new text credited the sp-y7n Note with describing campaign 031's fan-out proof working around the seam host-side; that sentence is in the sp-n8g amendment (docs/adr/0008 :447-449).

Cured in-branch at fd6a37e: the sentence now credits the sp-n8g amendment for the 031 description and the sp-y7n Note for answer_parent/3 being the explicit host door, plus one stray wrap the reviewer flagged as a tidy-up. Diff against origin/main is 189 insertions, 0 deletions. A fresh scope-frozen re-review follows.

[johnnyt]

Direction review pass 2 (scope-frozen re-review, head fd6a37e): UNQUALIFIED, 0 findings, 0 blocking, no new surface.

The pass-1 cure was verified from both sides and the misattribution is fixed rather than moved. Scope, append-at-end (189 insertions / 0 deletions), heading and status shape against the sp-n8g amendment, S1-S5 plus the worked example, no contradiction with ADR-0004 d6 / ADR-0008 d3 / ADR-0009 / ADR-0010, every code anchor present at the PR head with its 4e3e2c8 label, and the firewall scan clean with a control that fired.

Two non-qualifying advisories carry as residue rather than a cure (after UNQUALIFIED nothing is cured): this body's provenance section says 188 insertions where the cured diff is 189, and the section says "decision 5 below" where its other self-references say "clause N".

Merging under the campaign-SF039 grant: full gate not applicable (docs-only, per the CLAUDE.md authority table), CI green on fd6a37e, firewall clean with a constructed positive control, direction verdict UNQUALIFIED within budget.