docs: rewrite README as a library README (consumer ISendProxyManager API)

Covers what it is (per-field network value substitution, uniform/per-client/
per-entity), supported types (int/uint/fixed/bool/float/qangle/vector3) vs
passthrough (coord/normal/quantized/string/array), the consumer API with accurate
snippets (resolve in OAM, SetUniformInt, HookInt/Float/Bool/Vector + entity
variants, unhook), the callback thread-safety contract, the sp_example_* command
table, build/deploy + gamedata-nesting gotcha, AGPL + makesig attribution.

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