Unify the webform start and resume handlers into a single start-or-resume handler

orchestra_interaction_webform shipped two Webform handlers: one to
start a workflow from a submission, and one to resume a parked step on submit.
They carried nearly the same settings and could drift apart (for example
capturing the submission value map on start but not on resume), and a form that
both starts a run and is later reopened to modify it needed both handlers wired
in parallel.

Proposed resolution

Replace the two handlers with a single Orchestra: start or resume on
submit handler (orchestra_interaction), keyed on the binding
element's continuation handle:

• No handle (empty or absent binding element) and a workflow
  configured: the submission is a fresh start. It starts the workflow, binds the
  submission to the new instance as its source entity, and seeds the variables.
• Handle present (the form was reopened by the workflow): the
  submission is a step. It resumes the bound instance's parked token with the
  configured result and refreshes the stored submission id and value map.

Because one handler holds one set of settings, the start and resume sides can
no longer drift, and the "start then modify" loop needs only one handler. The
protected onResumed() extension point is preserved.

Changes

• New OrchestraInteractionHandler; the old
  OrchestraStartHandler and OrchestraResumeHandler are
  removed.
• Config schema merged to webform.handler.orchestra_interaction.
• orchestra_webform_example: the Request form's two handlers
  collapse to one orchestra_interaction handler (workflow
  request_validation, result modified, seeding
  request_sid, poster_uid, request).
• New documentation page docs/webform.md: a full configuration
  reference for the single-handler model (binding element rules, handler settings,
  scenarios, wait-node modes, chaining, gotchas, a worked example).
• Kernel tests rewritten as OrchestraInteractionHandlerTest (start,
  resume, modify loop, token validation, branch scope, expiry, tampering, loop
  reentry, no-op after advance); Functional tests updated.

Breaking change for consumers

This removes OrchestraStartHandler and
OrchestraResumeHandler from the module's public surface. A consumer
that subclassed either must extend OrchestraInteractionHandler
instead (the extension points are preserved). The known consumer,
yoyaku_webform, has its companion change in place.