feat: U5 land deterministic chacha20 KAT (RFC 8439 2.4.2)

- test/gen_chacha_kat.py emits KAT from RFC 8439 section 2.4.2 mapped to the impl's 64+64 counter/nonce layout; expected bytes cross-checked against pycryptodome (independent impl) and the official vector: 10f1e7e4d13b5915... byte-exact
- verdict: the 'intermittent chacha20 KAT FAIL' referenced in FUCK comments was a phantom — that test was never merged into this tree; the implementation itself matches RFC byte-for-byte
- selftest now runs KAT + page-MAC determinism checks at selftest=2; full SELFTEST PASS in QEMU (.build_tmp/qemu_serial_u5kat.log)

CODE/UTSM/test/utsm_selftest.d

@@ -5,7 +5,8 @@ test/utsm_selftest.o: test\utsm_selftest.c include\utsm\utsm.h \
   include\utsm\vmm.h include\utsm\linux_loader.h include\utsm\ipc_shm.h \
   ..\utsm-ipc\ipc_proto.h include\utsm\hypercall.h include\utsm\pckc.h \
   include\utsm\crypto.h include\utsm\log.h include\utsm\arena.h \
-  include\utsm\drr.h include\utsm\sched_ext.h include\utsm\sched.h
+  include\utsm\drr.h include\utsm\sched_ext.h include\utsm\sched.h \
+  test\chacha_kat.inc
 include\utsm\utsm.h:
 include\utsm\types.h:
 include\utsm\status.h:
@@ -28,3 +29,4 @@ include\utsm\arena.h:
 include\utsm\drr.h:
 include\utsm\sched_ext.h:
 include\utsm\sched.h:
+test\chacha_kat.inc: