machine/micropython/time/utime can't match MicroPython's positional-only C-function signatures: the compiler has no PEP 3102 `/` syntax

[begeistert]

What happens

MicroPython's own .pyi stubs (micropython-rp2-stubs, the parity suite's reference) mark
every C-implemented function and most method setters as positional-only, using PEP 3102's
/ marker: def sleep(seconds, /) -> None: .... This layer's real Python source can never
match that shape, because neither PyMCU frontend parses /:

• The hand-written C# parser (src/compiler/Frontend/Parser.cs, ParseParameters()) has no
  branch for TokenType.Slash; def f(a, /, b=0): is a hard parse error there today
  ("Expected parameter name", pointing at the /).
• The CPython-ast frontend (PYMCU_PY_PARSER=1, Frontend/PyParser/pymcu_translate.py)
  parses / without erroring, but silently merges posonlyargs into the ordinary
  positional list (no flag is ever emitted), so the restriction is a no-op even there.
• Param (src/compiler/Frontend/Ast.cs) has no positional-only flag at all, and call
  binding (src/compiler/IR/IRGenerator/Call.cs) matches keywords purely by name.

Since the default frontend is the C# parser, / cannot be used in this repo's source without
breaking the build for anyone not setting PYMCU_PY_PARSER=1.

What this blocks

tests/parity/ compares this layer's real (CPython-importable) function signatures against
the stub via inspect.signature, which correctly reports POSITIONAL_ONLY for stub params
before /. Every one of the symbols below differs from its stub only in that kind (name,
default and required-ness already agree, or agree once the harmless renames below are made);
none of them can be closed until the compiler accepts /.

• time.sleep, time.sleep_ms, time.sleep_us, time.ticks_add, time.ticks_diff
  (and the utime aliases of all five)

• micropython.const (stub param is named expr, ours value)

• micropython.native, micropython.viper (stub param is named _func, ours f)

• machine.enable_irq

• machine.UART.readinto (name already matches: buf)

• machine.UART.write (stub param is named buf, ours data)

• machine.UART.irq (added 2026-09-15, UART init/deinit/flush/sendbreak/txdone need new HAL primitives (PyMCU/PyMCU#451); irq() done #16: handler/trigger/hard
  are all positional-only in the stub, (self, handler=None, trigger=0, hard=False, /);
  names, defaults and required-ness already agree)

• machine.Pin.value, machine.Pin.mode, machine.Signal.value (their setter overload's
  x is positional-only, (self, x, /); the getter overload already matches exactly,
  closed 2026-09-15 in Pin.value()/Pin.mode()/Signal.value() encode "read" as the sentinel value 255, which collides with the legal value 255 #13)

• machine.PWM.duty_ns, machine.PWM.duty_u16, machine.PWM.freq (each already has a
  real no-arg getter and a one-arg setter @inline overload, matching arity; only the
  setter's positional-only value blocks an exact match)

• machine.time_pulse_us (also needs timeout_us widened past uint16: the stub's
  default is 1 000 000 µs (1 s, MicroPython's convention), which does not fit the
  16-bit parameter this HAL currently uses)

• machine.I2C/machine.SoftI2C's write, readinto, writeto, readfrom_into,
  writeto_mem, readfrom_mem_into (added 2026-09-15, I2C/SoftI2C stop= done; buffer-vs-heap shapes documented #14/I2C/SoftI2C primitives mostly done; init/deinit need PyMCU/PyMCU#455, writevto/I2CTarget remain #15:
  each matches the real stub in every param name, kind and default except the trailing /)

• machine.SPI.write, SPI.write_readinto, SPI.readinto (added 2026-09-15,
  I2C/SoftI2C stop= done; buffer-vs-heap shapes documented #14: same, blocked only by /)

Fix

Add / support to src/compiler/Frontend/Parser.cs (ParseParameters(), mirroring the
existing bare-* handling), a positional-only flag on Param, and the matching change in
pymcu_translate.py/PythonAstReader.cs so both frontends agree. This is a compiler-repo
change (PyMCU/PyMCU); tracked here because it blocks this layer's own parity suite
(tests/parity/, allowlisted tracked:#<this issue>).