refactor: collapse the typed value fan-out into one SpoofValue + SendProxyCallback

Addresses the ModSharp maintainer's 'callback hell' feedback. Replaces the 6 typed
PerClient*Proxy delegates with a single SendProxyCallback(client, entityIndex, ref
SpoofValue), and SpoofValue (Kind tag + factories Int/Float/Bool/Vector/String/Bytes +
typed As* accessors + Raw* for the dispatch) carries any spoofable value.

Counts: public delegate types 6→1; ISendProxyManager value methods 30→5 (Hook/Hook-ent/
SetUniform/SetUniform-ent/SendFake, each 6→1); SpoofEntry callback+value fields 13→3;
FieldSubstitution registration methods 20→4; the per-field dispatch 6-arm switch → one
callback invocation.

Dispatch bit-logic UNCHANGED — verified: seed SpoofValue from reg.Value, invoke the one
Callback, read intBits/vec/str/bytes back via Raw* (Float32 still via
SingleToInt32Bits(RawFloat)); the scratch-encode/emit block is byte-identical, only the
source is re-routed. IsValidEntity guards preserved. Builds 0 errors / 0 warnings.

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