* Add `jl_inside_heartbeat_thread()`

To allow `jl_safe_printf()` to determine whether it's being called
from the heartbeat thread.

* Write heartbeat thread output to the safe crash log

In `jl_safe_printf()`, we're already writing to the safe crash log
if we're in signal handler context (in addition to writing to
`stderr`). Now we do the same if we're in the heartbeat thread.

* Refactor JSON printing code

Concurrent `write()` calls to the same file descriptor should be
thread-safe, but can result in interleaving. Refactor the JSON
printing code used from `jl_safe_printf()` to assemble the message
into a buffer and use a single `write()` call.

* Add `jl_inside_heartbeat_thread()`

To allow `jl_safe_printf()` to determine whether it's being called
from the heartbeat thread.

* Write heartbeat thread output to the safe crash log

In `jl_safe_printf()`, we're already writing to the safe crash log
if we're in signal handler context (in addition to writing to
`stderr`). Now we do the same if we're in the heartbeat thread.

* Refactor JSON printing code

Concurrent `write()` calls to the same file descriptor should be
thread-safe, but can result in interleaving. Refactor the JSON
printing code used from `jl_safe_printf()` to assemble the message
into a buffer and use a single `write()` call.

* Add `jl_inside_heartbeat_thread()`

To allow `jl_safe_printf()` to determine whether it's being called
from the heartbeat thread.

* Write heartbeat thread output to the safe crash log

In `jl_safe_printf()`, we're already writing to the safe crash log
if we're in signal handler context (in addition to writing to
`stderr`). Now we do the same if we're in the heartbeat thread.

* Refactor JSON printing code

Concurrent `write()` calls to the same file descriptor should be
thread-safe, but can result in interleaving. Refactor the JSON
printing code used from `jl_safe_printf()` to assemble the message
into a buffer and use a single `write()` call.

Stdlib: Distributed
URL: https://github.com/JuliaLang/Distributed.jl
Stdlib branch: master
Julia branch: master
Old commit: cd92195
New commit: d439c24
Julia version: 1.14.0-DEV
Distributed version: 1.11.0 (Does not match)
Bump invoked by: @DilumAluthge
Powered by:
[BumpStdlibs.jl](https://github.com/JuliaLang/BumpStdlibs.jl)

Diff:
JuliaLang/Distributed.jl@cd92195...d439c24

```
$ git log --oneline cd92195..d439c24
d439c24 CI: Add a CI job that runs JET tests (#164)
d396a8c Fix some JET errors around matching methods for `send_connection_hdr(...)` and `send_msg_now(...)` (#180)
231da28 Fix some JET errors around matching methods for `kill(...)` and `process_exited(...)` (#172)
2adcd26 Rename one method of `run_work_thunk()` to `run_work_thunk_remotevalue()`; this fixes a JET error around matching methods for `run_work_thunk(...)` (#181)
1bc91f9 Fix a JET error regarding the existence of the local variable `reducer` at a certain point (#169)
b7c43b2 Fix a JET error around matching methods for `push!(...)` (#173)
d06aa73 Fix a JET error around matching methods for `getindex(...)` (#170)
0cf9910 Fix a JET error around matching methods for `read_worker_host_port(...)` (#171)
9f6459f Fix JET errors around matching methods for `lock(...)` and `unlock(...)` (#167)
9724553 Fix a JET error around matching methods for `shell_escape_wincmd(...)` (#174)
d65a996 Fix a JET error by narrowing the type signature of the `create_worker(x, y)` function (#175)
3ebddd3 avoid `Core.Box` in the package (#161)
2fe1aa4 Change `Distributed.cluster_manager` from a global non-constant `ClusterManager` to a global constant `Ref{ClusterManager}` (#177)
6649a94 Ensure correct type parameter of serialized RemoteChannel (#179)
871e3d7 Fix JET errors around matching methods for `send_msg_now(...)` (#165)
56fa9f2 Fix JET errors around matching methods for `send_msg(...)` (#166)
a6195c0 Bump actions/checkout from 5 to 6 (#155)
```

Co-authored-by: DilumAluthge <5619885+DilumAluthge@users.noreply.github.com>