Complete H.264 reference marking and stream capability checks

[antono3]

Changes

• Remove obsolete helper code and stale TODOs from the former library split.
• Parse 1, 2, and 4 byte AVC NAL prefixes, account for every slice in an MP4 sample, skip metadata-only samples, and reject slices that describe different pictures.
• Preflight SPS/PPS syntax, truncation, reference counts, bit widths, and fixed-array limits before calling the pinned H.264 parser; reject invalid crop dimensions, slice types, and duplicate parameter-set IDs.
• Resolve noncontiguous SPS/PPS IDs in both MP4 parsing and runtime decode command construction. Add a five-frame fixture with SPS/PPS ID 7 and tests for its parameter sets and slice references.
• Convert H.264 SPS levels to Vulkan's ordinal enum and reject streams above a GPU's reported maximum before session creation.
• Correct weighted-prediction header parsing; calculate H.264 POC types 0–2 and separate top/bottom counts; apply sliding-window and MMCO 1–6 reference marking, including long-term references and MMCO 5 renumbering.
• Select a GPU using the stream's level, padded coded extent, DPB/reference limits, output mode, and required image formats.
• Preserve custom H.264 SPS/PPS scaling lists in Vulkan session parameters; add an opt-in NV12 readback path and FFmpeg comparison script. Update the learning guide with direct source-line links and fixture instructions.

Verification

• Pinned V 0.5.2: all 3 software test files passed; release and trace executables built. The new tests include SPS/PPS truncation and single-bit mutations, invalid slice type, and nonzero parameter-set IDs.
• Linux GeForce GTX 1060: decoded NV12 matched FFmpeg byte for byte for the ID-7 fixture (5 frames), four-slice fixture (24), MMCO 5 Tandberg stream (300), and long-term-reference Sony stream (60). The Sony comparison first exposed a gray-output bug caused by missing custom scaling-list values; it matched after the fix. Reproduction commands are in PLATFORM_SUPPORT.md.
• On the Linux GTX 1060, X11 window captures matched FFmpeg within 2/255 per RGB channel at nine BT.601 limited-range color-bar pixels and eight BT.709 full-range color-bar pixels. Other color modes, full-frame output, and other drivers remain unverified.
• Checked 151 local Markdown links, including source-line targets; all resolve and direct targets were inspected.
• Release CI on current head 6a5ac21: Linux stable shared/static, pinned Linux V3, and Windows stable all passed. The advisory current-master lane is still running on this head; its previous run failed on generated C .output and StructureType errors.

Remaining validation

• Windows Vulkan Video hardware playback remains untested because no compatible machine is available.
• Other color modes, full-frame window output, and other GPU drivers remain unverified.
• SPS/PPS preflight catches truncation and bounded fields; it does not establish every semantic relationship in arbitrarily corrupted bitstreams.