✨ Bind a command's settled result so a document can decide on its exit code

[taras]

Story

As an executable-document author, I want to bind a command's process outcome —
its exit code, stdout and stderr — as a value, so the document decides what a
non-zero exit means instead of coercing the shell into hiding it.

Motivation

An ordinary exec block forwards stdout and stderr and raises on a non-zero
exit. That is right for a command whose failure is a failure. It is wrong for a
command whose failure is an answer.

scripts/bootstrap-npm-package.md in #308 asks npm what versions, dist-tags and
trusted publisher exist. For a package that does not exist yet — the normal
bootstrap case — npm exits nonzero and reports E404. The document currently has
no way to receive that process outcome as data, so its nine read-only calls use
shell coercion:

npm view {packageName} versions --json --registry {registry} 2>&1 || true

2>&1 merges the channels and || true discards the exit status. The
classifier then pattern-matches human-readable text. The workaround fails
closed, but it puts a security-relevant decision in the shell even though the
process already supplied separate channels and an exit status.

This is a language gap rather than a document problem. AGENTS.md prefers
expanding Executable.md capabilities over using JavaScript or shell machinery.

Architecture contract

The public syntax uses the existing exec and as vocabulary. It introduces
no new modifier or architecture term:

```bash exec as="versions"
npm view @executablemd/workflow versions --json --registry https://registry.npmjs.org
```

as="name" is a code-block binding annotation, not modifier middleware. It is
removed before modifier composition and binds the process outcome into the
ordinary eval binding environment:

{
 exitCode: number;
 stdout: string;
 stderr: string;
}

The object is a fresh, ordinary mutable binding value. Replay preserves its
field values, not its object identity. The internal CodeBlockResult.output
rendering field is not public binding data.

Grammar

• A bound command requires exactly one trailing, quoted as="name" annotation.
• The name uses the same ES-module-safe validateBindingName() contract as
  component invocation as, <Capture as>, and the other binding constructs.
• Missing, duplicate, unquoted, malformed or non-trailing annotations fail
  before a process starts.
• as on an executable code block is supported only with the exec terminal.
  An ordinary exec without as is unchanged.
• Only built-in timeout middleware may precede a bound exec, for example
  bash timeout=30s exec as="versions". Other built-in modifiers and custom
  modifiers are refused before a process starts. A timeout remains a failure;
  it does not become a process outcome.

Routing, retention and replay

PR #442 established the foreground boundary and merged as
7819058d5b2f979635d868e6d7e41c1f23448a87. A bound exec uses that boundary:

• Both stdout and stderr received at the per-exec boundary are buffered for the
  binding and neither channel is displayed.
• The binding receives exactly what trusted enclosing host stdio middleware
  allowed to reach that boundary.
• Binding is not retention authority. A transient host buffers both channels
  only for the current binding and does not add them to the durable exec record.
• When the trusted host selected retention, the existing exec event records both
  channels exactly once. Replay reconstructs the same binding values from that
  event without starting another process.
• The command keeps its existing durable exec identity and creates no wrapper
  effect or second process execution.

Outcomes and failures

• A process that starts and reaches an exit status binds its outcome. A non-zero
  status is data, does not raise, and continues even inside <Output>.
• Failure to start, timeout, cancellation, durability failure and infrastructure
  failure remain failures and create no binding.
• A bound block contributes no rendered output. Ordinary exec behavior is
  unchanged.

Bootstrap consumer

PR #308 contains nine 2>&1 || true occurrences, not seven: three initial
registry reads, three reads after elicitation, and three final verification
reads. All nine migrate to exec as="name" after this API is available.

npm uses a nonzero status for package absence and for unrelated registry,
authentication and network failures, so status alone must not authorize a
publish:

• exitCode === 0: parse stdout as the requested registry value.
• Nonzero: accept absence only from npm's structured JSON diagnostic when
  error.code === "E404".
• Every other nonzero or unreadable response stops before mutation.

The classifiers switch on exitCode and structured data rather than an E404
text regex. stdout and stderr remain separate; no shell redirection or status
coercion remains.

Acceptance

• A nonzero command binds mutable { exitCode, stdout, stderr }, displays
  neither channel, renders nothing and continues; a later ordinary eval reads
  the binding.
• The same bound command continues inside <Output>, while an ordinary failing
  exec remains a checked failure and prevents later work.
• A retained replay restores identical binding fields without another process
  start; a transient run supplies both fields live without granting retention.
• Process-start failure, timeout, cancellation, durability failure and
  infrastructure failure create no binding.
• Every invalid grammar or modifier combination is refused before execution.
• Specification, architecture and implementation describe the same grammar,
  value, routing, retention, replay and failure boundaries.
• After the API lands, ✨ Bootstrap an npm package as a real xmd run #308 removes all nine shell coercions and its classifiers
  follow the fail-closed structured-E404 rule above.

Delivery stack

1. Implement the core, architecture and executable-MDX specification change for
   ✨ Bind a command's settled result so a document can decide on its exit code #447 from main at or after the 🚿 Stream a foreground command and keep what the boundary received #442 merge commit
   7819058d5b2f979635d868e6d7e41c1f23448a87 in its own focused PR.
2. Rebase draft PR ✨ Bootstrap an npm package as a real xmd run #308 (currently 6f02b3f67ea723c4e0d065110a3a575a02662159
   on pre-🚿 Stream a foreground command and keep what the boundary received #442 base db62a71fcc628718d6d0171284ca525330875767) onto the
   landed ✨ Bind a command's settled result so a document can decide on its exit code #447 result.
3. Migrate ✨ Bootstrap an npm package as a real xmd run #308's bootstrap document and black-box tests before ✨ Bootstrap an npm package as a real xmd run #308 merges.

No synthetic merge of open heads is an implementation base. The #447 PR owns
the language contract; #308 owns its bootstrap consumer and integration suite.

Not this issue

• Stream foreground exec output and retain it only for explicit journals #441/🚿 Stream a foreground command and keep what the boundary received #442 decide foreground routing and host-selected retention. This issue
  consumes that boundary without changing its authority.
• No change to ordinary exec: a command whose nonzero status is a failure
  keeps failing unless its block explicitly uses as.