encryption: sealed channel (X448 + HKDF-SHA384), and neutralize dalek's SIMD by cfg

.cargo/config.toml

@@ -22,5 +22,31 @@
 # silicon) is a fifth opt-in mode — see § 7.1 of
 # .claude/knowledge/simd-dispatch-architecture.md. Reserved for the
 # release-binary distribution path; never the dev / CI default.
+# curve25519-dalek backend: SERIAL, deliberately.
+#
+# `ed25519-dalek` pulls in `curve25519-dalek`, whose build.rs AUTO-SELECTS its
+# own AVX2 backend whenever the target is x86_64 + 64-bit — which the
+# `-Ctarget-cpu=x86-64-v3` line below guarantees. That backend carries its own
+# raw intrinsics and its own `unsafe`: 57 `_mm*` intrinsic calls (35 in
+# `backend/vector/avx2/field.rs`, 22 in `backend/vector/packed_simd.rs`) under
+# 52 `unsafe` occurrences in the same two files — a SECOND unaudited SIMD
+# surface beside `ndarray::simd`, which is exactly what the matryoshka pattern
+# exists to prevent (cf. `vendor/chacha20`: "no raw intrinsics and no `unsafe`
+# here — all of that lives once, audited, inside `ndarray::simd`").
+#
+# Note this is a REACHABILITY question, not a porting one. Neutralizing that
+# surface does not require porting 57 call sites onto `ndarray::simd`; the whole
+# `vector` module is gated behind one cfg — `backend/mod.rs:42`,
+# `#[cfg(curve25519_dalek_backend = "simd")] pub mod vector;` — so the one line
+# below compiles the whole surface out.
+#
+# `serial` costs nothing we use: X25519's Montgomery ladder
+# (`montgomery.rs`) never references the vector backend at all, and Ed25519
+# signing is not on a hot path here. The vector backend serves ONLY Edwards
+# multi-scalar / variable-base multiplication (batch signature verification,
+# Ristretto protocols) — neither of which this crate performs.
+#
+# Read by curve25519-dalek's build.rs via CARGO_CFG_CURVE25519_DALEK_BACKEND.
+# Verify: cargo build -p encryption -v 2>&1 | grep curve25519_dalek_backend
 [target.'cfg(target_arch = "x86_64")']
-rustflags = ["-Ctarget-cpu=x86-64-v3"]
+rustflags = ["-Ctarget-cpu=x86-64-v3", "--cfg", "curve25519_dalek_backend=\"serial\""]