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/oracle_bench_test.go

@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+// Test-only benchmarks for comparing with the Rust port (scripts/bench.sh).
+
+package main
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+// Unlocking a key file: Argon2id for both slots.
+func BenchmarkOracleUnlock(b *testing.B) {
+	p := filepath.Join(b.TempDir(), "key")
+	if err := writeKeyFile(p, "bench", ""); err != nil {
+		b.Fatal(err)
+	}
+	for b.Loop() {
+		if _, err := unlockKeyFile(p, "bench"); err != nil {
+			b.Fatal(err)
+		}
+	}
+}