Migrate Hera->Artemis storage-attach to a real message round-trip

Hera still polls xHCI and sig-checks attached drives, but the storage
registration step (blk_subsys_attach_device(), now wrapped as the
BLK-ATTACH primitive) moves to Artemis's own dictionary, reached via
HERA-BLK-ATTACH-REQ/BLK-ATTACH-ACK (VM-EXEC, since Hera can't load her
own messaging.4th -- see the doc comment in repl.c). Identity birth
(Zuse genesis / WIREBIND) is deferred until the ack confirms storage
actually succeeded, instead of running synchronously underneath a
storage call that might fail ("wait for ack, safer for identity data").

Caught and fixed a real bug live during acceptance testing: Artemis's
ACK-APPEND-NUM fed a single-cell value into <# #S #> (which expects a
double-cell pair), causing a stack underflow the first time
HERA-BLK-ATTACH-REQ ran. Fixed with the same `0 SWAP` convention every
other numeric-append helper in this codebase already uses.

Verified booting clean to (zuse) ok> with no VM-EXEC errors on all
three architectures (amd64/aarch64/riscv64).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014Ec88YKxxhZGG1RNnune78

capsules/artemis/init.4th

@@ -500,3 +500,40 @@ MSG-CD-INIT
 2 MY-CH-ID !
 S" 2 COMMON-CH @ CH-ADD-MBR" S" Hermes" VM-EXEC
 STARTUP-BANNER
+
+Block 4856
+( Storage-attach ack builder. Hera sends "<dev-addr>          )
+( HERA-BLK-ATTACH-REQ"; reply is "<dev-addr> <ok?>            )
+( BLK-ATTACH-ACK" so identity birth only proceeds once        )
+( storage is confirmed (repl.c's BLK-ATTACH-ACK). Bob,        )
+( 2026-09-07.                                                  )
+CREATE ATTACH-ACK-BUF 64 ALLOT
+VARIABLE ATTACH-ACK-LEN
+: ACK-APPEND ( src-addr len -- )
+  DUP >R
+  ATTACH-ACK-BUF ATTACH-ACK-LEN @ +
+  SWAP CMOVE
+  ATTACH-ACK-LEN @ R> + ATTACH-ACK-LEN ! ;
+
+Block 4857
+: ACK-APPEND-CHAR ( char -- )
+  ATTACH-ACK-BUF ATTACH-ACK-LEN @ + C!
+  ATTACH-ACK-LEN @ 1+ ATTACH-ACK-LEN ! ;
+: ACK-APPEND-NUM ( n -- )
+  0 SWAP <# #S #> ACK-APPEND ;
+
+Block 4858
+( HERA-BLK-ATTACH-REQ ( dev-addr -- ): VM-EXEC'd by MSG-      )
+( DELIVER when Hera's request arrives. Runs BLK-ATTACH on     )
+( Artemis's own dictionary (real ACL gating, same as any      )
+( other message payload), then replies to Hera (idx 0).       )
+: HERA-BLK-ATTACH-REQ ( dev-addr -- )
+  0 ATTACH-ACK-LEN !
+  DUP ACK-APPEND-NUM
+  BL ACK-APPEND-CHAR
+  DUP BLK-ATTACH ACK-APPEND-NUM
+  DROP
+  BL ACK-APPEND-CHAR
+  S" BLK-ATTACH-ACK" ACK-APPEND
+  BLK-ATTACH-EVENT 2 0 ATTACH-ACK-BUF
+  ATTACH-ACK-LEN @ 0 MSG-SEND ;