Document Windows execution model

Author: mearvk

src/windows/README.md

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+# Windows Execution Model
+
+This package provides the Windows-side execution vocabulary for the Java 21 server/frontend reference architecture.
+
+The model is deliberately separated into seven concepts:
+
+- **Execution** — performs a controlled process launch and records its result.
+- **Program** — identifies an executable and constructs its argument vector.
+- **Directive** — expresses the policy under which execution is permitted.
+- **Citizen** — supplies a non-secret identity/accountability context.
+- **Call** — records who requested which program under which directive.
+- **Choice** — represents ordered executable alternatives and selects an available one.
+- **Structure** — binds the complete execution context into one immutable envelope.
+
+The implementation uses Java 21 records where the object is naturally immutable and a final class for the executable operation. It does not embed credentials or silently elevate privileges.
+
+For production hardening, process timeouts, Windows ACL checks, executable allow-lists, audit persistence, and explicit cancellation should be implemented by the surrounding service before unrestricted process execution is enabled.