feat(subs): child workflows + docs

[michael-0acf4]

Support for child workflows.

Solves MET-689 and MET-668.

Migration notes

Previously

    sub = SubstantialRuntime(backend)
    hello = sub.deno(file="workflow.ts", name="sayHello", deps=[])

    g.expose(
      # each function start, stop, result, ... holds a copy of the workflow data
       start_hello = hello.start(...),
       stop_hello = hello.stop()
    )

This approach relied on workflow files being referenced in each materializer, but the constructs were too restrictive to support something like mutation { results(name: "nameManuallyGiven") }.

We now have instead

    file = (
       WorkflowFile
           .deno(file="workflow.ts", deps=[])
           .import_(["sayHello"])
           .build()
    )

    # workflow data are refered only once
    sub = SubstantialRuntime(backend, [file])
    g.expose(
      start_hello = sub.start(...).reduce({ "name": "sayHello" }),
      stop = sub.stop()
    )

• The change comes with new or modified tests
• Hard-to-understand functions have explanatory comments
• End-user documentation is updated to reflect the change

[codecov]

Codecov Report

Attention: Patch coverage is 86.23188% with 57 lines in your changes missing coverage. Please review.

Project coverage is 77.09%. Comparing base (defae4c) to head (d7f5461).

Files with missing lines	Patch %	Lines
src/typegate/src/runtimes/substantial/agent.ts	67.54%	37 Missing ⚠️
src/typegate/src/runtimes/substantial.ts	87.50%	18 Missing ⚠️
tests/runtimes/substantial/common.ts	98.34%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #867      +/-   ##
==========================================
+ Coverage   77.01%   77.09%   +0.07%     
==========================================
  Files         149      149              
  Lines       17867    18129     +262     
  Branches     1737     1756      +19     
==========================================
+ Hits        13761    13976     +215     
- Misses       4083     4130      +47     
  Partials       23       23              

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