ncdb: derive coveritem flags from type defaults

The UCIS LRM defines IS_32BIT, HAS_GOAL, HAS_WEIGHT as structural
flags determined by cover type, not per-bin state. All CVGBIN items
have IS_32BIT|HAS_GOAL|HAS_WEIGHT (0x19); all others have IS_32BIT
(0x01). These are now stored in COVER_TYPE_DEFAULTS and applied
automatically during deserialization.

coveritem_flags.bin only stores flags that DIFFER from the type
default (exclusion flags, IS_BR_ELSE, IS_FSM_RESET, etc.). For
typical databases this means 0 entries → member not written.

Impact on merged.vdb benchmark (131K bins):
- coveritem_flags.bin: 264KB → 0 (not written)
- Read:  3.13s → 2.79s (-11%, ~340ms saved)
- Write: 2.71s → 2.63s (-3%, ~80ms saved)
- Merge: unchanged (fast path never touches flags)

Full test suite: 1002 passed, 0 failed

Author: mballance

src/ucis/ncdb/constants.py

@@ -67,21 +67,21 @@
 # Used by reader to reconstruct coveritem objects without per-item storage.
 
 COVER_TYPE_DEFAULTS: dict = {
-    CoverTypeT.TOGGLEBIN:  (0, 0, 1),
-    CoverTypeT.STMTBIN:    (0, 0, 1),
-    CoverTypeT.BRANCHBIN:  (0, 0, 1),
-    CoverTypeT.CONDBIN:    (0, 0, 1),
-    CoverTypeT.EXPRBIN:    (0, 0, 1),
-    CoverTypeT.FSMBIN:     (0, 0, 1),
-    CoverTypeT.CVGBIN:     (0, 1, 1),
-    CoverTypeT.DEFAULTBIN: (0, 0, 1),
-    CoverTypeT.IGNOREBIN:  (0, 0, 1),
-    CoverTypeT.ILLEGALBIN: (0, 0, 1),
-    CoverTypeT.BLOCKBIN:   (0, 0, 1),
-    CoverTypeT.COVERBIN:   (0, 0, 1),
-    CoverTypeT.ASSERTBIN:  (0, 0, 1),
-    CoverTypeT.PASSBIN:    (0, 0, 1),
-    CoverTypeT.FAILBIN:    (0, 0, 1),
+    CoverTypeT.TOGGLEBIN:  (0x01, 0, 1),
+    CoverTypeT.STMTBIN:    (0x01, 0, 1),
+    CoverTypeT.BRANCHBIN:  (0x01, 0, 1),
+    CoverTypeT.CONDBIN:    (0x01, 0, 1),
+    CoverTypeT.EXPRBIN:    (0x01, 0, 1),
+    CoverTypeT.FSMBIN:     (0x01, 0, 1),
+    CoverTypeT.CVGBIN:     (0x19, 1, 1),
+    CoverTypeT.DEFAULTBIN: (0x01, 0, 1),
+    CoverTypeT.IGNOREBIN:  (0x01, 0, 1),
+    CoverTypeT.ILLEGALBIN: (0x01, 0, 1),
+    CoverTypeT.BLOCKBIN:   (0x01, 0, 1),
+    CoverTypeT.COVERBIN:   (0x01, 0, 1),
+    CoverTypeT.ASSERTBIN:  (0x01, 0, 1),
+    CoverTypeT.PASSBIN:    (0x01, 0, 1),
+    CoverTypeT.FAILBIN:    (0x01, 0, 1),
 }
 
 # ── Scope-type → implicit child cover type mapping ────────────────────────

src/ucis/ncdb/coveritem_flags.py

@@ -17,6 +17,7 @@
 
 from .varint import encode_varint, decode_varint
 from .dfs_util import dfs_scope_list
+from .constants import COVER_TYPE_DEFAULTS
 
 _VERSION = 1
 _COVER_ALL = 0xFFFFFFFF
@@ -32,7 +33,9 @@ def serialize(self, db) -> bytes:
         for scope in scopes:
             for ci in scope.coverItems(_COVER_ALL):
                 flags = ci.getCoverFlags()
-                if flags != 0:
+                ct = ci.getCoverData().type
+                default_flags = COVER_TYPE_DEFAULTS.get(ct, (0, 0, 1))[0]
+                if flags != default_flags:
                     entries.append((global_ci_idx, flags))
                 global_ci_idx += 1
 
@@ -83,4 +86,8 @@ def deserialize(self, data: bytes, db) -> None:
                 if entry_pos < len(entries) and entries[entry_pos][0] == global_ci_idx:
                     ci.setCoverFlags(entries[entry_pos][1])
                     entry_pos += 1
+                else:
+                    ct = ci.getCoverData().type
+                    default_flags = COVER_TYPE_DEFAULTS.get(ct, (0, 0, 1))[0]
+                    ci.setCoverFlags(default_flags)
                 global_ci_idx += 1

src/ucis/ncdb/scope_tree.py

@@ -265,7 +265,8 @@ def _read_toggle_pair(self, data: bytes, offset: int, parent, counts_iter):
         # Create the two implicit TOGGLEBIN coveritems
         for (bin_name, count) in ((TOGGLE_BIN_0_TO_1, count_0to1),
                                    (TOGGLE_BIN_1_TO_0, count_1to0)):
-            cd = CoverData(CoverTypeT.TOGGLEBIN, 0)
+            cd = CoverData(CoverTypeT.TOGGLEBIN,
+                           COVER_TYPE_DEFAULTS.get(CoverTypeT.TOGGLEBIN, (0,0,1))[0])
             cd.data = count
             scope.createNextCover(bin_name, cd, None)
 
@@ -344,7 +345,8 @@ def _read_regular_scope(self, data: bytes, offset: int, parent, counts_iter):
             ci_name_ref, offset = decode_varint(data, offset)
             ci_name = self._st.get(ci_name_ref)
             count = next(counts_iter, 0)
-            cd = CoverData(child_cover_type, 0)
+            default_flags = COVER_TYPE_DEFAULTS.get(child_cover_type, (0, 0, 1))[0]
+            cd = CoverData(child_cover_type, default_flags)
             cd.data = count
             if at_least_override is not None or (child_cover_type and
                     COVER_TYPE_DEFAULTS.get(child_cover_type, (0,0,1))[1] != 0):