Documentation for transient federates.

[edwardalee]

This PR adds documentation for transient federates.

[Copilot]

Pull request overview

This PR adds a new documentation page describing “transient federates” (federates that can join/leave a running federation) and links/integrates that content into existing federation, termination, polyglot, glossary, and target-reference docs.

Changes:

• Add a new “Transient Federates” guide page and include runnable C/Python example programs.
• Cross-link transient-federate behavior from distributed execution, termination, polyglot docs, and target-language reference.
• Add glossary entries and a sidebar entry so the new page is discoverable.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
docs/writing-reactors/transient-federates.mdx	New documentation page describing transient federates, joining/leaving, hot swap, and limitations.
docs/writing-reactors/termination.mdx	Adds a note clarifying how to stop only the calling federate (for transient federate departure).
docs/writing-reactors/polyglot.mdx	Notes that transient federates are supported for Polyglot due to C/Python support.
docs/writing-reactors/distributed-execution.mdx	Adds a brief pointer from coordinated start to transient federates.
docs/sidebars.ts	Adds the new doc page to the “writing-reactors” sidebar.
docs/reference/target-language-details.mdx	Documents effective start-tag API and “stop only this federate” APIs for C/Python.
docs/glossary/glossary.mdx	Adds glossary definitions for federate/federation/persistent/transient federates.
docs/assets/code/py/src/TransientFederates.lf	Adds Python example program used by the new documentation page.
docs/assets/code/c/src/TransientFederates.lf	Adds C example program used by the new documentation page.

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[Copilot]

Pull request overview

Copilot reviewed 8 out of 9 changed files in this pull request and generated no new comments.

Suppressed comments (3)

docs/reference/target-language-details.mdx:2979

• LF_FED_PACKAGE_DIRECTORY is generated only for federated C programs, but this bullet is under “Libraries Available in All Programs.” Qualify it as federated-only so non-federated users are not led to reference an undefined macro.

  - LF_FED_PACKAGE_DIRECTORY: A string giving the full path to the directory that is the root of the federate package (normally, the directory above the `src` directory appended with `fed-gen/name`, where `name` is the name of the federation).

docs/writing-reactors/transient-federates.mdx:185

• The RTI does not run the replacement binary; the operator launches that process, and the RTI accepts its connection. The current wording incorrectly places arbitrary-code execution inside the RTI’s process/trust boundary.

A hot-swapped binary is still arbitrary code that the RTI will run as that federate. Treat the ability to launch a replacement the same way you treat the ability to start the federation: restrict who can obtain the federation ID, credentials, and host access.

docs/writing-reactors/transient-federates.mdx:52

• Remove the duplicated space between “three” and “federates.”

The following program, adapted from the Lingua Franca regression tests, has three  federates. `Up` sends integers every two seconds. `Middle` is transient: it forwards those integers to `Down` and, after four inputs, leaves the federation. `Down` has its own timer, so it keeps executing whether `Middle` is present or not.

[edwardalee]

I'm going ahead and merging this to get the release out.