feat: U3 @sealed optional segment marking - phase 2 complete

- UTSM_SEG_F_UNSEALED: plaintext direct-store segments (zero crypto overhead) while keeping capability checks, dirty tracking, intent/commit log and seqlock protocol; default remains sealed
- linker.ld: .sealed output section with __sealed_start/__sealed_end symbols for future static sealed data
- selftest: unsealed segment stores plaintext verifiably + read-back, full SELFTEST PASS (.build_tmp/qemu_serial_u3sealed.log)
- phase 2 (UTSM/UTRW) now complete in ROADMAP_INSANE

CODE/UTSM/utrw/read.c

@@ -20,6 +20,12 @@ int utsm_read(utsm_capability cap, u64 offset, void *dst, u64 len) {
         return status;
     }
 
+    /* U3: @unsealed 段 —— 明文直存，0 加解密开销（capability 校验照走） */
+    if (desc->flags & UTSM_SEG_F_UNSEALED) {
+        memcpy(dst, desc->cipher_base + offset, len);
+        return UTSM_OK;
+    }
+
     utsm_key_material key;
     status = utsm_pckc_get_or_derive(cap.segment_slot, desc, &key);
     if (status != UTSM_OK) {

CODE/UTSM/utrw/read.d

@@ -0,0 +1,14 @@
+utrw/read.o: utrw\read.c include\utsm\utrw.h include\utsm\types.h \
+  include\utsm\status.h include\utsm\capability.h include\utsm\segment.h \
+  include\utsm\uuid.h include\utsm\dirty.h include\utsm\pckc.h \
+  include\utsm\config.h include\utsm\crypto.h
+include\utsm\utrw.h:
+include\utsm\types.h:
+include\utsm\status.h:
+include\utsm\capability.h:
+include\utsm\segment.h:
+include\utsm\uuid.h:
+include\utsm\dirty.h:
+include\utsm\pckc.h:
+include\utsm\config.h:
+include\utsm\crypto.h:

CODE/UTSM/utrw/write.c

@@ -21,6 +21,18 @@ int utsm_write(utsm_capability cap, u64 offset, const void *src, u64 len) {
         return status;
     }
 
+    /* U3: @unsealed 段 —— 明文直存，0 加解密开销；dirty/seqlock/log 照走，
+     * checkpoint 路径对明文页同样适用（页 MAC 语义由 DRR 侧统一处理）。 */
+    if (desc->flags & UTSM_SEG_F_UNSEALED) {
+        drr_log_write_intent(cap.segment_slot, offset, len);
+        desc->writer_seq++;
+        memcpy(desc->cipher_base + offset, src, len);
+        utsm_mark_dirty(desc, offset, len);
+        desc->writer_seq++;
+        drr_log_write_commit(cap.segment_slot, offset, len);
+        return UTSM_OK;
+    }
+
     utsm_key_material key;
     status = utsm_pckc_get_or_derive(cap.segment_slot, desc, &key);
     if (status != UTSM_OK) {

CODE/UTSM/utrw/write.d

@@ -0,0 +1,15 @@
+utrw/write.o: utrw\write.c include\utsm\utrw.h include\utsm\types.h \
+  include\utsm\status.h include\utsm\capability.h include\utsm\segment.h \
+  include\utsm\uuid.h include\utsm\dirty.h include\utsm\pckc.h \
+  include\utsm\config.h include\utsm\crypto.h include\utsm\drr.h
+include\utsm\utrw.h:
+include\utsm\types.h:
+include\utsm\status.h:
+include\utsm\capability.h:
+include\utsm\segment.h:
+include\utsm\uuid.h:
+include\utsm\dirty.h:
+include\utsm\pckc.h:
+include\utsm\config.h:
+include\utsm\crypto.h:
+include\utsm\drr.h: