…oice (#1544)

…#1491)

* feat: allow start() to be called directly inside workflow functions

Add 'use step' to start() so it can be called directly from workflow
code. The SWC compiler strips the function body in workflow mode and
replaces it with a step proxy. When called from a workflow:

1. The workflow function reference is serialized via WorkflowFunction
   reducer (serializes { workflowId })
2. start() executes in the step context with full Node.js access
3. The returned Run is serialized via WORKFLOW_SERIALIZE and deserialized
   back in the workflow VM
4. Run getters (.status, .returnValue, etc.) are 'use step' getters
   that each execute as separate steps

Also re-exports start from @workflow/core/runtime/start in api-workflow.ts
instead of using a throwing stub, adds e2e tests for startFromWorkflow
(with hook communication) and fibonacciWorkflow (recursive composition).

* fix(next): don't copy package step files in deferred builder to avoid duplicate classes

Files belonging to packages (detected by walking up to find a
package.json with a name field) are imported via relative path
instead of being copied to __workflow_step_files__/. Copying creates
a second module instance which breaks JS native private field (#)
brand checks when the runtime creates instances from one copy and
the step handler accesses fields from the other.

* fix(next): only skip copying package files that are serde classes, not all package step files

Regular package step files (like fetch) must still be copied to ensure
the SWC loader registers them. Only serde class files from packages are
excluded from copying since those define classes with JS native private
fields (#) that break when duplicated.

* fix(next): generate thin wrappers for package serde step files instead of full copies

For package files that define serde classes (like Run), generate a thin
wrapper that imports the original class and registers steps/classes from
the manifest. This avoids duplicating the class definition (which breaks
JS native private field brand checks) while still registering all step
functions and the class in the serialization registry.

Regular package step files (like fetch) are still copied as before.

* fix(next): use forceStepModeFiles to transform package serde files in step mode

Instead of copying package serde+step files (which creates duplicate
classes with #private brand check issues) or generating fragile wrappers,
add the original file paths to a shared forceStepModeFiles set. The
loader checks this set and transforms those files in step mode directly,
so the SWC plugin generates proper step registrations on the original
class — no duplication, no reimplemented registration logic.

* fix(next): use step mode for all files with step/serde patterns, not just copies

The loader now selects step mode for any file that has 'use step'
directives or serde patterns, regardless of whether it's a deferred
step copy. Step mode is a superset of client mode — the only addition
is step registry IIFEs, which are harmless for non-step consumers.

This means package serde+step files (like Run) no longer need to be
copied to get step registrations. They're imported directly in the
step route and the loader transforms the original file in step mode.
One class instance, no duplication, no wrapper generation.

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Co-authored-by: Nathan Rajlich <n@n8.io>