Phase 0: skeleton, streaming encryption (spike A)

Repository skeleton (app id ng_backup, AGPL-3.0-or-later), KeyRing with
KDF subkeys, keyed blob ids and Argon2id key wrapping, and StreamCipher
(secretstream XChaCha20-Poly1305, framed, bounded memory).

Spike A: 1 GiB round trip with memory_limit=64M, peak < 2 MiB, ~160/122
MiB/s; tampering, truncation, wrong key/passphrase are rejected.

.gitignore

@@ -0,0 +1,5 @@
+/vendor/
+/node_modules/
+/js/
+/build/
+/spikes/out/