docs: add HOW_IT_WORKS.md — newcomer-facing "how SendProxy works" article

A plain-English walkthrough for someone new to the project: why per-field
spoofing is hard on Source 2 (pack-once + per-client bit-copy, no SendProp
proxy), the two substitution paths (uniform encoder hook vs per-client
redirect-src), the encoder buckets, the API + targeting modes, the two common
gotchas (must re-dirty to show; values mirrored across pawn+controller), safety
& lifecycle, and what it can't cleanly do. Links the deeper RE docs as
reference. README points newcomers here first.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>