Fix nondeterministic export output and several correctness bugs

[xmx-emm]

Summary

Re-exporting the same asset could produce output files that differ by a few bytes/pixels on every run. I tracked this down to several independent sources of nondeterminism, plus a few adjacent correctness bugs found along the way. All are fixed in this PR; each fix is small and local.

Nondeterministic export output

• cpakfile.h getStarPakData(): the destination buffer was allocated uninitialized and the stream read result was never validated. Requested mip sizes are page aligned, so reads near the end of a (opt)starpak can come up short, leaving random heap bytes in the tail of the exported texture data - a few random pixels that change on every run. The buffer is now zero initialized so short reads produce deterministic zeroes.
• rmax.cpp RMAXExporter::ToFile(): the 16 MiB scratch buffer was uninitialized, while the writer advances with IALIGN16 after every data section and rounds the final file size up to 16 bytes. The alignment gaps were written to disk as heap garbage, so every exported .rmax differed. The buffer is now zero initialized.
• dx.cpp ExportAsPng() / ExportAsDds(): exports run on a thread pool, and two tasks can write the same output path (e.g. two materials sharing a texture with "Export Material Textures" enabled, or two selected assets sharing a dependency with "Export asset dependencies" enabled). Concurrent writers interleave within a single file and corrupt it differently each run. File writes are now serialized per output path through a small hashed mutex pool.
• modeldata_qc.cpp: s_QCMaxVerts was a file-scope global mutated by concurrent QC exports, so a parallel export could write another model's $maxverts value into the QC file. It is now a local passed by reference through QC_ParseStudioBodypart / QC_ParseStudioLOD.

Shared texture mutation

• dx.cpp ExportAsPng() converted the texture to B8G8R8A8 in place via ConvertToFormat, which deletes and replaces the underlying scratch image. Atlas textures (rawTxtr / convertedTxtr) are shared between preview and both export paths, so exporting PNG first silently changed what a later DDS export wrote (converted data instead of the original block-compressed format), and two threads exporting the same texture raced on the delete/replace. The conversion now happens into a temporary ScratchImage, leaving the source texture untouched.

One-pixel offset in ui image atlas slices

• ui_image_atlas.cpp: posX/posY were truncated while width/height were rounded to nearest (+ 0.5f). When minX * atlasWidth lands just below a whole number (e.g. 99.99997), the slice origin came out one pixel off. The position now uses the same round-to-nearest as the size; this resolves the existing // should this have the same rounding as below ? todo study this note.
• Also fixed the always-true uiImage->height == uiImage->height comparison in the preview's main-texture check (should compare against uiAsset->height).

Smaller fixes

• ui_image.cpp: the memcpy_s destination sizes for transparent tile fills did not subtract tileOffset; the BC7 slice-pitch assert compared in the wrong direction (BC1 branch had it right).
• texture.cpp: the mip table "Type" column sort compared a.type against b.level.
• imgui_utility.cpp: secondary progress bar names were passed to ImGui::Text as a format string; now TextUnformatted.
• jsonutils.h/.cpp: the copyright line contained an encoding-mangled character (U+FFFD). On systems with a non-western ANSI codepage (e.g. 936), MSVC raises C4819 which the project's warnings-as-errors turns into a build failure - main currently does not compile on such systems. Replaced with (c).

Not addressed here (worth a follow-up): RTech::DecompressStreamedBuffer passes the decompressed size as the available input byte count to OodleLZDecoder_DecodeSome, which can over-read the compressed input buffer. Fixing it requires a signature change across many call sites.

Test plan

• Full rebuild of Release_CI succeeds (warnings-as-errors, /W4)
• Exporting the same asset twice now produces byte-identical files (streamed textures, ui image atlas PNG/DDS, rmax)
• DDS atlas export after PNG atlas export retains the original block-compressed format
• Parallel "export all" with material textures + dependencies enabled produces stable output