S0240-cli-pipe-1: terminate cleanly when a consumer closes stdout

[NSchatz]

A developer who pipes any cosyte command into head, a pager they quit, or any
early-exiting consumer now gets a clean termination under a documented exit
code instead of a silent success or a stack trace.

The measurement that chose the code

Task 1 of the spec is a measurement, taken on real spawned processes at the
previous pin before anything changed. Node v22.23.2:

case	exit	stderr
no error listener on process.stdout	1	unhandled 'error', Error: write EPIPE, ten stack frames
cosyte parse msg.hl7 (single write)	0	empty, and nothing was delivered
cosyte parse big.ndjson --ndjson	70	value-free CLI_OUTPUT_WRITE_FAILED
cosyte-mcp with a request in flight	1	unhandled 'error', full stack trace naming node_modules paths

The platform error identity, which the research file could not reproduce and
carried as unverified: an Error with code: 'EPIPE', syscall: 'write',
errno: -32, delivered as an 'error' event on the process.stdout socket.
The write callback receives the same error, which is what makes the
single-write path detectable without waiting on an event race.

The code

A new IOERR: 74, the value sysexits.h assigns EX_IOERR. Additive: no
existing code is renamed, repurposed or removed, and no existing code's
published meaning changes. 70 stays "an unexpected exception, i.e. a bug",
which is precisely why a closed pipe cannot live there: nothing went wrong
inside the CLI, and reporting a consumer's own choice as a defect is the
"documented lie" shape this repository already names elsewhere. The one
condition that moves onto the new code is the closed-consumer write failure
itself, which the spec authorises explicitly.

What changed

• src/core/exit-codes.ts gains IOERR: 74, published on every surface that
  states the contract: README.md, docs-content/reference-commands.md,
  docs-content/concepts-archetype.md and docs-content/quickstart.md.
• src/commands/parse.ts: the CLI_OUTPUT_WRITE_FAILED raise moves off
  EXIT.SOFTWARE onto EXIT.IOERR. One constant, the only src/commands
  change.
• src/bin/cosyte.ts: the single-result write now goes through the same
  closed-stream check the record stream uses and waits for the platform's
  acknowledgement, so a result that never reached the consumer overrides the
  code the command had resolved to. A run whose output never arrived is never
  a success.
• src/bin/cosyte-mcp.ts and the cosyte mcp subcommand terminate quietly
  under the same code when the client's end of stdout is gone, by closing the
  transport's input rather than calling process.exit(), which the platform
  documents as truncating pending stream writes.
• Both bins hear stderr's own 'error' event, so a closed diagnostic channel
  cannot turn a quiet termination into an unhandled error.
• test/pipe-close.test.ts spawns real processes and closes their real
  streams: single-record, multi-record, nothing-delivered, closed stderr, and
  both routes to the MCP server. Every case carries its own timeout, and a
  negative control replays the unguarded platform behaviour so the value-free
  assertions are a verdict rather than an absence.
• The two contract-locking tests admit the new code: the golden matrix drives
  an invocation that reaches it, and the docs guard binds EX_IOERR to 74
  on every shipped surface.

src/mcp/ tool logic is untouched, and no release is published (a changeset
is included; changeset publish is not run).

Gates

pnpm test 733/733 · pnpm test:coverage 98.22% statements, 93.5% branches ·
pnpm typecheck · pnpm lint · pnpm format:check · pnpm phi-scan ·
pnpm check:no-emdash · pnpm check:no-internal-refs ·
pnpm check:agent-notes · pnpm build · pnpm smoke · pnpm attw all green.

Spec: work/specs/S0240-cli-pipe-1/spec.md

[NSchatz]

Fix loop for impl-gate finding F1 (work/specs/S0240-cli-pipe-1/verdict-impl-1.md), pushed as 808fb33.

F1: the single-result write already waited for the platform's acknowledgement, but the record stream did not. deps.writeStdout only consulted the stdoutOpen flag, which the platform flips asynchronously, so a stream short enough to enqueue every line before the closed-stream error arrived resolved to EXIT.OK, and the bin's delivery check then short-circuited on result.stdout === "". cosyte parse f --ndjson into a departed consumer exited 0 with a success summary over zero delivered bytes.

Fix, in src/bin/cosyte.ts only: both write paths go through one emit, every chunk is owed an acknowledgement, and the run may not resolve until each has come back. An undelivered run reports the value-free CLI_OUTPUT_WRITE_FAILED under 74 and the command's own summary is withdrawn with the code it computed.

Advisory F2 taken: the closed-consumer record-stream cases now run at one and two records (the counts where the defect is reachable) and assert the absence of a success summary, and scripts/smoke.mjs asks the same question of the packaged bin under plain node, after build in the same required job. Mutation-checked against the unfixed adapter: 2 suite cases and 4 smoke checks go red.