Sync docs: ARCHITECTURE, PROJECT, PROJECT_PLAN

- ARCHITECTURE.md §12: add mciasdb, mciasgrpcctl, internal/grpcserver,
  proto/, and gen/ to the directory structure diagram
- ARCHITECTURE.md §17: replace buf generate references with protoc;
  the implementation uses protoc + protoc-gen-go + protoc-gen-go-grpc
  invoked via go generate ./... (proto/generate.go)
- PROJECT_PLAN.md §7.1: replace buf.yaml/buf.gen.yaml acceptance
  criteria with the protoc-based go:generate approach actually used
- PROJECT_PLAN.md §7.6: clarify that gen/ is committed to the repo
  (not gitignored); only the binary /mciasgrpcctl is excluded
- PROJECT.md: replace scrypt with Argon2id (the actual algorithm);
  remove the redundant Argon2 suggestion line

PROJECT.md

@@ -38,7 +38,8 @@ Performance is secondary, and can be tuned later.
 
 ## Technical details
 
-+ User passwords will be stored using scrypt.
++ User passwords will be stored using Argon2id (PHC format), meeting
+  OWASP 2023 recommended parameters (time=3, memory=64 MiB, threads=4).
 + The service account tokens and user/password authentication can be
   used to obtain a JWT, if that is appropriate.
 + All authentication events should be logged.
@@ -46,8 +47,7 @@ Performance is secondary, and can be tuned later.
   git.wntrmute.dev/kyle/goutils for logging etc.
 + The database should be sqlite.
 + Modern cryptography should be used. Preference should be given to
-  Ed25519 as the public algorithm for signatures, for example. Consider
-  the use of Argon2 for password hashes.
+  Ed25519 as the public algorithm for signatures, for example.
 
 ## Interfaces