feat: sealed memory early init + verify - UTSM-E01..E04 very-early panic

- NEW core/sealed.c: utsm_sealed_early_init() runs right after utsm_init:
  empty .sealed section -> UTSM-E01 panic; no root key -> UTSM-E02;
  establishes BLAKE2b(root_key) MAC over [__sealed_start,__sealed_end).
- utsm_sealed_verify(): MAC mismatch -> UTSM-E03; verify-before-init ->
  UTSM-E04. Wired into utsm_selftest (U3+ step).
- .sealed region now non-empty by construction: sealed brand record
  "DESHAB-SEALED-MEMORY-V1-DEAICUP" lives in section(".sealed").
- Before this commit nothing ever referenced __sealed_start/__sealed_end:
  the sealed section was never registered, MACed or verified.
- QEMU verify: [SEAL] region=0x20 bytes, MAC established, selftest OK,
  boot reaches wizard, zero spurious panics.

[STUB] P0/UTSM/core/sealed.c | 剩余STUB命中=101（违规=0） | “封缄即落款，空区即自爆” | 已提交=<this>

CODE/UTSM/test/utsm_selftest.c

@@ -100,6 +100,13 @@ int utsm_selftest_run(void) {
             return UTSM_ERR_INVALID;
         }
         log_info("[UTSM] selftest page MAC determinism ok");
+
+        /* ==== U3+: 封缄内存 .sealed 区 MAC 校验（UTSM-E03 路径自证） ==== */
+        {
+            extern void utsm_sealed_verify(void);
+            utsm_sealed_verify();   /* 不符即 panic，返回即通过 */
+            log_info("[UTSM] selftest sealed MAC verify ok");
+        }
     }
 
     /* ==== U4: 密文页 dump —— 串口输出密文/明文 hex 对照 ==== */