Add optional NT98513 MIPS diagnostic tools

[dpiers]

Adds an optional build path for three source-owned NT98513 Linux MIPS diagnostics: descriptor metadata, one raw control transaction, and a typed active-sensor query. BUILD_NT98513_DIAGNOSTICS defaults to OFF; enabling it adds a self-contained tools/nt98513 subtree and a separate NT98513Diagnostics install component.

The subtree also builds standalone with an installed compatible compiler. It checks the actual little-endian MIPS32r2/O32 soft-float compiler target, supplies freestanding syscall entries, and installs the three tools with their README and MIT notice. Ordinary configuration performs no downloads. Build and private-staging commands are in tools/nt98513/README.md.

Validation of the submitted build code:

• Standalone and enabled parent builds with CMake 3.25.1 and MIPS GCC 12.2.0; both produce identical diagnostic executables. Static ELF attributes, selected entry calls, source/object dependencies and staged contents checked.
• Twelve installed CLI cases, each normally and under QEMU 7.2.22 tracing: help/no arguments, invalid input and absent devices. Exact output/exit checks passed; absent-device paths make the expected failed open and no ioctl.
• Option-OFF ARM base/candidate builds produce byte-identical ipctool and ipcinfo, including both existing ipcinfo size configurations (all: 47,356 bytes; none: 38,492 bytes).
• The twenty diagnostic C/header/assembly files are unchanged from their separately reviewed native/sanitizer/MIPS implementations. A post-build README-only correction clarifies the documented commands' CMake 3.13 minimum; compilation code is unchanged.

Draft status: there is no camera available for validation. These tools do not establish compatible kernel/RTOS services, a bootable OpenIPC target or capture. Descriptor metadata provides no frame-payload lease. The sensor query describes mutable RTOS selection and clears shared command staging; raw control commands can change camera state. Device calls may block indefinitely. This contribution adds no autostart or firmware installation and is separate from identification PR #245.