Convert C macros to strongly-typed C# enums (MacroEnumPostProcessor)

[Ruslan-B]

Problem

Many FFmpeg constants are defined as C macros (e.g. AV_CODEC_FLAG_*, SWS_*, AV_CH_*). These are generated as individual constants but not grouped into enums, making the API less discoverable and type-unsafe.

Solution

Add a MacroEnumPostProcessor that groups macros by prefix into C# enums:

• AV_CODEC_FLAG_ → CodecFlags
• AV_CODEC_FLAG2_ → CodecFlags2
• SWS_ → SwsFlags (already exists but could be enriched)
• AV_CH_ → ChannelFlags
• AV_PIX_FMT_FLAG_ → PixFmtFlags
• 48+ total mappings

Complexity: High

New processor class. Needs careful prefix matching, flag detection, type hints (int vs ulong). Breaking API change — constants become enum members.

Reference: SuRGeoNix/Flyleaf.FFmpeg.Generator MacroEnumPostProcessor.cs