Track MicroPython RP2 stub API parity gaps

[begeistert]

This issue was the single umbrella every tests/parity/ allowlist entry pointed at while the
parity suite was new. It is now an index: every one of the 218 symbols the suite flagged
against micropython-rp2-stubs has been sorted into either a documented design/hardware
limit (docs/limitations.md, quoted in tests/parity/allowlist.toml) or one of the
per-cause issues below, and no allowlist entry points at this issue anymore.

Documented limits (not tracked as gaps; see docs/limitations.md)

• network.* -- no radio hardware on this class of AVR chip
• rp2.* -- no PIO/DMA hardware on this class of AVR chip
• machine.ADC's ATTN_*/WIDTH_*/CORE_*/ADCBlock/ADCWiPy -- ESP32/WiPy specific
• machine.Pin's ALT_*/DRIVE_*/drive/ANALOG/OPEN_DRAIN/PULL_HOLD/level IRQs --
  RP2040 pin-mux and drive-strength concepts this GPIO does not have
• machine.I2S/RTC/SDCard/USBDevice/mem32/mem_backup/bootloader -- peripherals
  this chip does not have
• machine.UART's CTS/RTS/IDLE/INV_RX/INV_TX/IRQ_BREAK/IRQ_RX/IRQ_RXIDLE/
  IRQ_TXIDLE -- no hardware flow control or break/idle IRQ source
• micropython's runtime-introspection helpers (mem_info, qstr_info, stack_use,
  heap_lock/heap_unlock, opt_level, kbd_intr, alloc_emergency_exception_buf,
  RingIO, Const_T) and asm_thumb/asm_xtensa -- no interpreter/heap at runtime, and
  no ARM/Xtensa target
• time/utime's gmtime/localtime/mktime/time/time_ns -- no RTC peripheral
• machine.PWM.init's stub ... defaults -- a stub-authoring convention, not a real default

Tracked gaps (one issue per distinct cause)

• machine/micropython/time/utime can't match MicroPython's positional-only C-function signatures: the compiler has no PEP 3102 / syntax #12 -- / (PEP 3102 positional-only) is unsupported by both compiler frontends; blocks
  time/utime/micropython/several machine signatures from ever matching exactly
• Pin.value()/Pin.mode()/Signal.value() encode "read" as the sentinel value 255, which collides with the legal value 255 #13 -- Pin.value()/Pin.mode()/Signal.value()'s sentinel 255 collides with the legal
  value 255
• I2C/SoftI2C stop= done; buffer-vs-heap shapes documented #14 -- I2C/SoftI2C/SPI read methods need a caller-owned buffer where MicroPython
  returns a list/bytes object, and I2C has no stop= repeated-start control
• I2C/SoftI2C primitives mostly done; init/deinit need PyMCU/PyMCU#455, writevto/I2CTarget remain #15 -- I2C/SoftI2C raw start/stop/write/readinto primitives,
  readfrom_mem/writeto_mem, and I2CTarget (slave mode) are not implemented
• UART init/deinit/flush/sendbreak/txdone need new HAL primitives (PyMCU/PyMCU#451); irq() done #16 -- UART.init/deinit/flush/irq/sendbreak/txdone are not implemented
• reset_cause() is not implemented: MCUSR needs a startup-stub capture in the HAL #17 -- reset_cause()/soft_reset() are not implemented though MCUSR backs them
• bitstream()/dht_readinto() need new HAL primitives (PyMCU/PyMCU#450), not thin wrappers #18 -- bitstream()/dht_readinto() are not implemented
• Pin.pull() needs PyMCU/PyMCU#448 (HAL getter); Pin.irq() blocked on #12 #19 -- Pin.init(value=...) and a Pin.pull() getter are missing
• Timer.init's three keyword-only overloads are blocked on PyMCU/PyMCU#447 (keyword args refused once >1 @inline overload exists) #20 -- Timer.init()'s shape doesn't match any one of the stub's three overloads
• machine.SPI accepts none of MicroPython's constructor forms, and SoftSPI is absent #4 (pre-existing) -- machine.SPI's constructor and SoftSPI -- covers
  SPI.init/CONTROLLER/LSB/MSB/SoftSPI

Original snapshot, for history

Initial results before allowlisting: 293 generated parity cases, 74 passing, 218 unexpected
failures, 1 skipped alias-only uasyncio stub case. That 218 is now the count sorted above.