system/zbus: Port the Zephyr zbus message bus to NuttX

[JorgeGzm]

Summary

This PR ports the Zephyr RTOS zbus (a many-to-many message bus with
typed channels and decoupled observers) to NuttX, preserving the
original declarative API (ZBUS_CHAN_DEFINE, ZBUS_LISTENER_DEFINE,
ZBUS_SUBSCRIBER_DEFINE, zbus_chan_pub/read/notify, ...) so that
existing Zephyr application code and documentation translate directly.
Original zbus by Rodrigo Peixoto (Apache-2.0); copyright preserved in
the derived files.

Ported features: listeners (synchronous callbacks), subscribers
(queue of channel references), message subscribers (ordered message
copies), async listeners (callback on the LP work queue), a deferred
ISR-safe publisher (ZBUS_ISR_PUBLISHER_DEFINE / zbus_isr_pub),
runtime observers, per-observation notification masks, observer
enable/disable, message validators, channel user data, publish
statistics, lookup by name/id and channel/observer iteration.

Everything is built on native NuttX primitives, with no compatibility
shim layer:

Zephyr	NuttX
k_sem channel lock + priority boost (HLP)	sem_t + native CONFIG_PRIORITY_INHERITANCE
k_msgq / k_fifo + net_buf pools	kernel message queues (file_mq_*), lazy-opened; mq payload copy replaces net_buf entirely
k_work	work_queue() (LP queue)
SYS_INIT	lazy init via pthread_once()
iterable sections	include/nuttx/iterable_sections.h (companion PR)
k_timeout_t	milliseconds, CLOCK_MONOTONIC deadlines (ZBUS_NO_WAIT / ZBUS_FOREVER)

Board integration (why one linker-script line matters here)

In Zephyr, zbus works on every board out of the box because boards have
no linker scripts: a single common per-arch linker template already
includes the shared common-rom.ld/common-ram.ld fragments where the
zbus iterable sections are collected. In NuttX each board owns its .ld,
so an adopting board needs one of:

• two #include <nuttx/linker/common-{rom,ram}.ld> lines in its board
  script (done for linum-stm32h753bi, the first adopter, in the
  companion PR), or
• CONFIG_ZBUS_LINKER_INSERT=y (zero-touch INSERT AFTER mode, with
  the MEMORY-region constraint documented in the companion PR).

This is documented in the Kconfig help, in README.rst and in the
Sphinx page added by the companion PR.

Also included:

• examples/zbus (CONFIG_EXAMPLES_ZBUS): a runnable demo with one channel,
  one listener, one subscriber, runtime masking.
• testing/zbus (CONFIG_TESTING_ZBUS): a cmocka suite, 16 tests
  covering the full API surface: pub/read/listener/subscriber,
  multi-channel index isolation, validators, message subscribers
  (ordered copies), bit-exact float/double payload delivery
  (sensor-style message with a float-math validator rejecting
  non-finite samples), claim/finish/notify, masks, enable/disable,
  runtime observers (error paths included), async listeners (bursts),
  ISR publisher, from_name/from_id, iteration, accessors, and
  timeout/overflow semantics.

Not ported (documented in the "Not ported" section of the docs):
multi-domain proxy agent (experimental upstream), direct publishing from
ISRs (the deferred zbus_isr_pub helper covers the use case), the
priority-boost/HLP scheme (superseded by native priority inheritance)
and the net_buf pool machinery (unnecessary with mq payload copies).

Impact

• New optional application (CONFIG_ZBUS, default n); no impact
  when disabled.
• Requirements: FLAT build (uses kernel-side file_mq_* so queues
  survive the creating task, since mqd_t is a per-task fd in NuttX);
  CONFIG_MQ_MAXMSGSIZE >= sizeof(pointer) + CONFIG_ZBUS_MSG_SUBSCRIBER_MAX_MSG_SIZE when message subscribers are
  enabled (checked and documented in Kconfig); board linker integration
  as described above.
• Build process: Make and CMake supported.
• Documentation: README.rst here; full Sphinx documentation (with
  the upstream zbus diagrams, Apache-2.0) lands with the companion
  nuttx PR.
• License: Apache-2.0, derived from Zephyr zbus; original copyright
  retained.

Testing

Host: Ubuntu 24.04.4 x86_64, Arm GNU Toolchain 13.2.rel1
(arm-none-eabi-gcc 13.2.1).

Target: linum-stm32h753bi:zbus (STM32H753BI; board configuration
added by the companion nuttx PR): CONFIG_ZBUS with all options
enabled (message/async/ISR observers, runtime observers),
CONFIG_EXAMPLES_ZBUS, CONFIG_TESTING_ZBUS. Flashed via ST-LINK V3;
console on the ST-LINK VCP.

nsh> uname -a
NuttX 13.0.1-RC0 a1b0941bec Aug 20 2026 21:17:15 arm linum-stm32h753bi

Example (zbus): 5 messages published; listener correctly skips the
masked message #4; listener notified before the subscriber (observer
priority order):

zbus: publishing 5 messages to acc_chan
zbus:  listener: x=1 y=10 z=100
zbus:  subscriber: x=1 y=10 z=100
...
zbus: listener masked
zbus:  subscriber: x=3 y=30 z=300
zbus: listener unmasked
zbus:  subscriber: x=4 y=40 z=400
zbus:  listener: x=5 y=50 z=500
zbus:  subscriber: x=5 y=50 z=500
zbus: done

cmocka suite: 16/16 passing, run twice in the same boot (guards
against the lazy-init/fd-lifetime regression class):

nsh> cmocka_zbus_test
[==========] tests: Running 16 test(s).
...
[ RUN      ] test_float_payload
[       OK ] test_float_payload
...
[==========] tests: 16 test(s) run.
[  PASSED  ] 16 test(s).

(The could not notify observer: -42 line during test_timeouts is the
expected -ENOMSG queue-overflow path being exercised.)

Builds: Make (make -j) and CMake (cmake -GNinja + ninja) both
OK for the target config; _zbus_* iterable-section symbols verified in
the map on both. Stock build with CONFIG_ZBUS disabled: no zbus
sections/symbols (no-op). tools/checkpatch.sh -g on the commit: all
checks pass.

The companion nuttx PR adds a linum-stm32h753bi:zbus board
configuration reproducing this exact run
(./tools/configure.sh linum-stm32h753bi:zbus); since that defconfig
enables Kconfig symbols introduced here, the two PRs should land
together.