docs(technical-debt): record dup and dup2 limits

docs/dev/TechnicalDebt.md

@@ -323,6 +323,7 @@ Throughout the project, every function at every stage either **does exactly what
 - [sigaltstack](../../core/libs/prx/libkernel/System/src/Signals.cpp) (libkernel) records each thread's alternate signal stack and reports it back, following FreeBSD's `kern_sigaltstack` (`EINVAL` for flags other than `SS_DISABLE`, `ENOMEM` below `MINSIGSTKSZ` 2048), but handlers never run on it, so `SS_ONSTACK` and `EPERM` are never reported
 - [libSceUlt queues](../../core/libs/prx/libSceUlt/Export.cpp) use preallocated host storage instead of the supplied guest work area; its existing size formula and the 512-byte object layouts remain unverified. Waiting-queue pool thread limits and waiter priority ordering are not implemented. Finalization wakes blocked queue callers with `ULT_ERROR_STATE`.
 - [Thread cancellation](../../core/libs/prx/libkernel/Pthread/src/Thread.cpp) (libkernel) follows FreeBSD libthr (states and types 0/1 and 0/2, `PTHREAD_CANCELED` 1, the `_pthread_cleanup_info` node layout), not measured on the console. A request is acted on only in `scePthreadTestcancel`, the condition variable, `sem` and `scePthreadSem` waits, `scePthreadJoin` and the sleeps, and their POSIX names; file and socket calls are not cancellation points. A sleep is not interrupted: the thread exits when it ends. The asynchronous type acts like the deferred one, except at the two set calls.
+- [dup and dup2](../../core/libs/prx/libkernel/File/src/Stdio.cpp) (libkernel) throw for the combinations they do not model: `dup2` between a guest socket and a host descriptor in either direction, and on Windows duplicating a directory descriptor. Guest sockets share one socket object between the duplicates and the descriptor numbers come from the guest socket counter, not the lowest free number as in FreeBSD `kern_dup`. The errno values for descriptor exhaustion are not verified against the console.
 - [MIMG `tfe` and `lwe`](../../core/shader/recompiler/RdnaDecoder/src/RdnaImageOpDecoder.cpp) never write the status VGPR after the data. On hardware it is written only when a texel fetch hits an unmapped page of a partially resident texture, or, for `lwe` on sample/gather, when the LOD is below the T# `MIN_LOD_WARN`; texture memory is always resident here, the LOD warning (texture-streaming feedback, like the T# `MIN_LOD_WARN` itself) is never reported, and `lwe` on `image_get_lod` throws.
 - [Packed horizontal gathers](../../core/shader/recompiler/SpirvBackend/src/SpirvImageEmitter.cpp) (`image_gather4h_pck`, MIMG opcode 98): the four window texels are read raw and concatenated into a little-endian bitstream, with DMASK bit i writing stream word i to the next destination register; measured on RDNA2 (gfx1031) at every edge: the anchor is floor(x-0.5) with elements at -1, 0, +1 and +2 clamped horizontally into the width, the row is floor(y), negative rows clamp to row 0, rows at or past the height return zeros, sampler addressing modes do not affect the window, and formats wider than 32 bits per texel write zeros in every register. The emitter fetches each window texel at LOD 0 and packs its bits like `image_load_pck`, so the sampler's filter, addressing and LOD state are not used; formats whose bits are not recoverable from the view and D16 data throw. The anchor and row multiply the normalized coordinate by the image size without contraction. The stream rule was measured for R8_UNORM, RG8_UNORM and RGBA8; only RGBA8 has an execution test. `image_gather8h_pck` (opcode 99) never returns texel data on this hardware - across dmasks 0x1 to 0xFF including the upper nibble, RGBA8, RGBA16 and RGBA32 formats, 1D, 2D and 3D descriptors and normalized and unnormalized sampling it only zero-fills popcount(dmask) registers (sentinel-prefilled destinations) - so it decodes but throws at translation; its eight-texel semantics remain unmeasured.
 - [waitpid](../../core/libs/prx/libkernel/System/src/Process.cpp) (libkernel) returns -1 with `ECHILD` (`EINVAL` for unknown options): nothing here creates a child process; child processes would need a real `waitpid`.