Consolidate HID report descriptor parsing across backends

[Youw]

Problem

HIDAPI currently has several private implementations that decode or walk HID report descriptors:

• libusb/hid.c has its own item walker for Usage Page/Usage extraction.
• PR Properly determine libusb read size for large reports (Fixes #274) #728 adds libusb/hidapi_libusb_report_descriptor.h to calculate maximum Input/Output/Feature report sizes.
• linux/hid.c independently implements report-item decoding and Usage Page/Usage extraction.
• netbsd/hid.c contains another closely related implementation.

These routines duplicate short/long item decoding, bounds checks, global-item state, Usage handling, and malformed-descriptor behavior. Keeping the new libusb parser under the backend directory also makes reuse by the hidraw and NetBSD backends unnecessarily difficult.

Windows' hidapi_descriptor_reconstruct.c performs the inverse operation—reconstructing a descriptor from preparsed data—so it may remain separate, but shared item definitions or test data could still be evaluated.

Proposal

Introduce a private common report-descriptor module, for example under src/ or another backend-neutral internal directory, and migrate reusable decoding logic into it.

The common layer should:

• Decode HID short and long items safely.
• Expose enough parser state or callbacks to derive Usage Page/Usage and per-report-ID sizes.
• Keep transport-specific descriptor retrieval in each backend.
• Remain an internal implementation detail with no public ABI additions.
• Compile as both C and C++ under HIDAPI_BUILD_AS_CXX.
• Preserve each backend's existing fallback and malformed-descriptor behavior.

Acceptance criteria

• libusb, Linux hidraw, and NetBSD reuse the common descriptor walker where practical.
• Duplicated byte/item decoding helpers are removed.
• Existing backend behavior remains covered by tests.
• The Windows fixture corpus and focused malformed/boundary cases provide shared parser coverage.
• The eventual location and naming of hidapi_libusb_report_descriptor.h no longer imply that generic parsing belongs exclusively to libusb.

Related to #728.

Related to #796.

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