unix: Add zephyr_ble variant with soft timer and machine.Timer

[andrewleech]

Summary

Adds a zephyr_ble Unix variant with pthread-based soft timer infrastructure and machine.Timer support. This is the first component of Zephyr BLE Unix port integration -- the BLE defines are present but commented out pending the Zephyr BLE unix port implementation.

The timer backend uses a dedicated pthread that sleeps on a condition variable and calls soft_timer_handler() when timers expire. A recursive mutex shared with MICROPY_PY_PENDSV_ENTER/EXIT provides thread-safe access to the timer heap. SIGUSR1 interrupts the main thread's blocking syscalls (via MICROPY_SCHED_HOOK_SCHEDULED) so scheduled callbacks are processed promptly during time.sleep() and similar.

Testing

Built and tested on Linux x86_64. machine_soft_timer.py test passes. Periodic and one-shot timer modes verified with correct tick counts across multiple runs. Standard variant test suite confirms no regressions from the conditional changes to main.c and gccollect.c.

machine_timer.py (which tests hard=True callbacks) segfaults because hard callbacks execute directly in the timer thread, which cannot safely call into the MicroPython VM. This is an inherent limitation of the pthread approach and matches the expected behavior -- hard callbacks require ISR-level execution on the main thread.

Generative AI

I used generative AI tools when creating this PR, but a human has checked the code and is responsible for the description above.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.49%. Comparing base (e4920d6) to head (5bdc38c).
⚠️ Report is 52 commits behind head on review/unix-soft-timer.
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Additional details and impacted files

@@                    Coverage Diff                     @@
##           review/unix-soft-timer      #33      +/-   ##
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+ Coverage                   98.45%   98.49%   +0.03%     
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  Files                         175      176       +1     
  Lines                       22635    23417     +782     
==========================================================
+ Hits                        22286    23064     +778     
- Misses                        349      353       +4     

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[github-actions]

Code size report:

Reference:  esp32/boards/SEEED_XIAO_ESP32C6: Add new XIAO board definition. [2dc2e30]
Comparison: unix: Add zephyr_ble variant with soft timer and machine.Timer support. [merge of 5bdc38c]
  mpy-cross:    +0 +0.000% 
   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
      esp32:    +0 +0.000% ESP32_GENERIC
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32