docs: update README for the unified SpoofValue API + drop SM-port framing

The README still described the old 6 typed PerClient*Proxy delegates + typed SetUniform
overloads + framed the project as a SourceMod port. Updated to the current ModSharp-native
unified API: one SendProxyCallback(client, entityIndex, ref SpoofValue) + SpoofValue
factories/accessors, with all code examples rewritten. Added SendFake + SetForceResend,
a Documentation index (HOW_IT_WORKS / REVERSE_ENGINEERING / FINDING_SIGNATURES /
FORCE_RESEND / MERGE_READINESS), and cross-platform + live-application status notes.

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