[Weidong Cui (wdcui)]

This PR adds terminal job control. The broker core now tracks a foreground process group for the controlling terminal's session and sends terminal events such as Ctrl-C to that group instead of the first root's group. Processes in the session query and change it through new process-group requests that take a handle to the terminal and report generic errors. The Linux shim implements tcgetpgrp, tcsetpgrp, and tcgetsid on top, mapping those errors to Linux errnos. Unit and end-to-end tests are included.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/enum_variant_added.ron

Failed in:
  variant ProcessError:NotControllingTerminal in /home/runner/work/litebox/litebox/litebox/src/process.rs:64
  variant ProcessError:NoSuchProcessGroup in /home/runner/work/litebox/litebox/litebox/src/process.rs:67
  variant ProcessError:NotInSession in /home/runner/work/litebox/litebox/litebox/src/process.rs:70

--- failure enum_discriminants_undefined_non_unit_variant: enum's variants no longer have defined discriminants due to non-unit variant ---

Description:
An enum's variants no longer have well-defined discriminant values due to a tuple or struct variant in the enum. This breaks downstream code that accesses discriminants via a numeric cast like `as isize`.
        ref: https://doc.rust-lang.org/reference/items/enumerations.html#assigning-discriminant-values
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/enum_discriminants_undefined_non_unit_variant.ron

Failed in:
  enum ProcessGroupResponse in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:630

--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/enum_variant_added.ron

Failed in:
  variant ProcessGroupResponse:GetForegroundGroup in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:636
  variant ProcessGroupResponse:SetForegroundGroup in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:638
  variant ProcessGroupResponse:Failed in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:642
  variant ProcessGroupRequest:GetForegroundGroup in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:390
  variant ProcessGroupRequest:SetForegroundGroup in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:399