Docs for the Rust version; benchmarks; build.sh fixes

README, CLAUDE.md, man pages, the user guide and its screenshot script
now describe the Rust build and the intentional behaviour changes, which
docs/MIGRATION.md lists. Go and Rust micro-benchmarks for the same
operations, and scripts/bench.sh for end-to-end comparison. build.sh
exports its tree root to the per-target jobs and keeps logs out of the
output directory; riscv64 musl builds use getrandom's raw syscall
backend, since zig's musl getrandom never reaches the kernel there.

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

oracle/go/internal/stream/oracle_bench_test.go

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+// Test-only benchmarks for comparing with the Rust port (scripts/bench.sh).
+
+package stream
+
+import (
+	"bytes"
+	"io"
+	"testing"
+)
+
+// STREAM seal and open of 64 MiB, one goroutine (the sequential path).
+func BenchmarkOracleStream64MiB(b *testing.B) {
+	key := make([]byte, 32)
+	pt := make([]byte, 64<<20)
+	var ct bytes.Buffer
+	b.SetBytes(int64(len(pt)))
+	for b.Loop() {
+		ct.Reset()
+		w, _ := NewEncryptWriter(key, &ct)
+		w.Write(pt)
+		w.Close()
+		r, _ := NewDecryptReader(key, bytes.NewReader(ct.Bytes()))
+		io.Copy(io.Discard, r)
+	}
+}