Records a neutral discriminator for exit reasons

[johnnyt]

Records ADR-0015 at Status proposed. sui-2s4 asked whether
session.terminated's reason should stop being bare inspect/1 output.
This PR answers it as a decision record and ships nothing else: no code, no
docs/wire-format.md edit, no changelog fragment, no implementing bead.

Provenance

• Cut from origin/main at 2071776, the squash-merge of sui PR Records a wire vocabulary for error reasons #74
  (sui-4lr, ADR-0014). ADR-0014 and its README index row were already on
  main when this branch was created, so the ADR-0014 row is inherited
  rather than merged around, and no rebase-over-sibling resolution was
  needed. This record is ADR-0015; ls docs/adr and the open-PR list
  were both checked for a number collision.
• A sibling worker is active on lib/, assets/, and mix.exs for
  sui-wqr. This branch touches none of them.

Touched files

File	Change
docs/adr/0015-a-neutral-discriminator-for-exit-reasons.md	new, 503 lines
docs/adr/README.md	+1 index row, after ADR-0014's

git diff origin/main -- docs/adr/ shows zero removed lines.

What the record decides

Add one field, kind, carrying a language-neutral lowercase token for the
exit reason's class, and leave reason byte-identical as the human overflow.

• Purely additive: every existing message stays byte-for-byte the same, the
  format version stays 1, and the projection position table does not move.
  The rejected alternative - mapping :normal / :shutdown /
  {:shutdown, _} / :killed to bare strings in place - is a changed
  value under an existing key, which ADR-0005's must-ignore rule does not
  cover; the record says so explicitly rather than waving at "additive".
• The golden fixture does not move: test/support/trace/two_state.jsonl
  contains zero session.terminated messages (verified, not assumed).
• kind is never projected; reason keeps its existing unconditional
  redaction. A projected stream therefore gains an outcome it cannot be
  told today.

Consistency with ADR-0014

Stated as its own decision section, with a side-by-side table. There is no
divergence to declare: ADR-0014 kept inspect/1 as a fallback field on the
explicit condition that a neutral token sit beside it, and cited this bead's
third acceptance criterion as the shape it was satisfying. ADR-0015 adopts
the same kind derivation rule verbatim and the same
inspect/1-beside-a-token split for the field ADR-0014 took the precedent
from, which closes that record's open coupling rather than opening a new one.
The only asymmetry is ADR-0014's class key, absent here because
session.terminated has one payload shape rather than two.

Two things flagged rather than buried

• The OTP crash shape {exception, stacktrace} classifies as "unknown"
  under the shared derivation rule, so the most informative failure gets the
  least specific token. Recorded as the record's weakest point, with the
  additive escape hatch named.
• The bead's first acceptance criterion selects the option this record
  rejects. sui-2s4 was written in 2026-08 as an implementation bead and
  asks for bare lowercase strings under reason itself. Flipping this
  record's Status also re-scopes that criterion, and the record's Notes say
  so.

A named alternative the operator may prefer (a curated OTP-reason allowlist
with an "other" token, so {:bad, 1} does not read as a recognised reason)
is recorded in the Notes and argued against rather than omitted.

Verification

• /wurk:verify --unattended found an empty backlog (no plan stage ran), so
  the substitute pass applies: every claim was machine-checked by a script.
• All 27 file:line cites in the record resolve against this branch's HEAD;
  two were wrong on the first draft and were corrected.
• Status line reads proposed; the record contains no Status: accepted.
• ADR number unique against docs/adr/ and against the open-PR list (empty).
• Working tree carries only the two files above.
• The bead's own acceptance criteria are implementation criteria and stay
  deferred to the implementing bead, which is filed on acceptance and not
  before.

Gate: not run - docs-only. gate.rb reports applicable: false
("no changes under lib/, test/, config/, mix.exs, mix.lock,
assets/, docs/wire-format.md"). CI replays the full gate on the PR.

Status: proposed - the flip is the operator's, and follows as its own
docs-only PR under the same review gate.

Refs: sui-2s4

[johnnyt]

Direction-agent verdict (campaign 027): UNQUALIFIED, 0 findings - merges AS PROPOSED; the accept flip is the operator's. Scope 503 added / 0 removed plus one index row; shape matches ADR-0014; Status proposed; the ADR-0005 and ADR-0014 consistency claims verified; every cite resolves; terminology scan exit 1 with a positive control. For the flip: the record openly rejects the bead's first acceptance criterion (Option A, mapping in place) in favour of an additive kind key (Option B), so accepting it also re-scopes that criterion. Non-qualifying tidy-ups: the 'eight lines below' cite gap is sixteen; 'verbatim' should read 'identical in effect'.