Reorganize platforms directory by platform instead of by type

[Copilot]

Platform-specific code was split across platforms/{platform}.py (data) and platforms/checkers/{platform}.py (checker classes), making it unclear which code belonged together.

Changes

Before:

platforms/
├── windows.py, darwin.py, posix.py          # Data (system_paths, invalid_chars)
└── checkers/
    └── windows.py, darwin.py, posix.py      # Checker classes

After:

platforms/
├── windows/
│   ├── checker.py                           # WindowsPathChecker
│   └── paths.py                             # system_paths, invalid_chars, reserved_names
├── darwin/
│   ├── checker.py                           # DarwinPathChecker
│   └── paths.py                             # system_paths, invalid_chars
└── posix/
    ├── checker.py                           # PosixPathChecker
    └── paths.py                             # system_paths, invalid_chars

Import Updates

In checker.py factory function:

# Before
from .platforms.checkers.windows import WindowsPathChecker

# After
from .platforms.windows.checker import WindowsPathChecker

In platform checker modules:

# Before
from ..windows import invalid_chars, system_paths

# After
from .paths import invalid_chars, system_paths

All platform-specific code now colocated by platform. No functional changes.

Original prompt

checker.py is getting a bit big. It contains classes for specific platforms and it might be better to use the platforms subpackage to contain all the relevant pltform specific code - but we would nee ot be careful of circular imports just moving the existing classes to the existing submodules within the platforms subpackages. Consider the best way to separate out the platform specific code to separate modules/packages within the platforms subpackage.

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