Support process groups and sessions across processes

[Weidong Cui (wdcui)]

This PR adds Linux process groups and sessions spanning LiteBox processes. A new broker ProcessGroup protocol family is backed by broker-core membership state: children inherit their parent's group and session on fork, and setpgid and setsid follow Linux's same-session, session-leader and existing-group rules. Signal requests can now target a single process, a process group, or every other process. In the Linux shim, the PR implements setpgid, getpgid, getpgrp, setsid and getsid (including in vfork children), kill(0), kill(-pgid) and kill(-1) through the broker (a group signal that includes the caller is queued before kill returns), and wait4(0) and wait4(-pgid) filtering by group membership. A multi-process runner e2e test covers these paths.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure constructible_struct_adds_field: struct exhaustively constructible through public API adds field ---

Description:
A pub struct that could be exhaustively constructed with a literal using only public API has a new pub field, breaking existing exhaustive literals.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field SendSignalRequest.target in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/signal.rs:33

--- 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 BrokerResult:ProcessGroup in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:365
  variant BrokerOperation:ProcessGroup in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:102

--- failure struct_pub_field_missing: pub struct's pub field removed or renamed ---

Description:
A publicly-visible struct has at least one public field that is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/struct_pub_field_missing.ron

Failed in:
  field process_id of struct SendSignalRequest, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/d5df8f676ed8d814f7090697a6c9fd5973eeb366/litebox_broker_protocol/src/signal.rs:21

--- failure enum_no_repr_variant_discriminant_changed: enum variant had its discriminant change value ---

Description:
The enum's variant had its discriminant value change. This breaks downstream code that used its value 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_no_repr_variant_discriminant_changed.ron

Failed in:
  variant SyscallRequest::Getuid 91 -> 96 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2602
  variant SyscallRequest::Geteuid 92 -> 97 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2603
  variant SyscallRequest::Getgid 93 -> 98 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2604
  variant SyscallRequest::Getegid 94 -> 99 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2605
  variant SyscallRequest::Sysinfo 95 -> 100 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2606
  variant SyscallRequest::CapGet 96 -> 101 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2609
  variant SyscallRequest::GetDirent64 97 -> 102 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2613
  variant SyscallRequest::SchedGetAffinity 98 -> 103 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2618
  variant SyscallRequest::SchedYield 99 -> 104 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2623
  variant SyscallRequest::Futex 100 -> 105 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2624
  variant SyscallRequest::Execve 101 -> 106 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2627
  variant SyscallRequest::Umask 102 -> 107 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2632
  variant SyscallRequest::Prctl 103 -> 108 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2635
  variant SyscallRequest::Alarm 104 -> 109 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2638
  variant SyscallRequest::Pause 105 -> 110 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2641
  variant SyscallRequest::SetITimer 106 -> 111 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2642
  variant SyscallRequest::GetITimer 107 -> 112 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2647
  variant SyscallRequest::Statx 108 -> 113 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2651
  variant SyscallRequest::TimerfdCreate 109 -> 114 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2658
  variant SyscallRequest::TimerfdSettime 110 -> 115 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2662
  variant SyscallRequest::TimerfdGettime 111 -> 116 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2668