graph/db: refactor CachedEdgePolicy to use explicit boolean fields

Replace MessageFlags and ChannelFlags bitfields in CachedEdgePolicy
with explicit boolean fields to improve clarity and support both v1
and v2 channel updates:

- Replace MessageFlags with HasMaxHTLC boolean.
- Replace ChannelFlags with IsNode1 and IsDisabled booleans.
- Update NewCachedPolicy to extract these fields version-appropriately:
  - For v1: derive from MessageFlags and ChannelFlags bits.
  - For v2: derive from policy.SecondPeer and policy.DisableFlags.

Update all call sites that used method calls IsNode1() and IsDisabled()
to instead access the fields directly. This includes:
- graph_cache.go: policy direction and disable checks
- unified_edges.go: HasMaxHTLC and IsDisabled checks
- Tests: policy construction and assertions

This refactoring improves readability by making the cached policy's
state explicit rather than encoded in bitfields, and enables seamless
support for both gossip protocol versions.

routing/unified_edges_test.go

@@ -30,7 +30,7 @@ func TestNodeEdgeUnifier(t *testing.T) {
 		FeeProportionalMillionths: 100000,
 		FeeBaseMSat:               30,
 		TimeLockDelta:             60,
-		MessageFlags:              lnwire.ChanUpdateRequiredMaxHtlc,
+		HasMaxHTLC:                true,
 		MaxHTLC:                   5000,
 		MinHTLC:                   100,
 	}
@@ -39,7 +39,7 @@ func TestNodeEdgeUnifier(t *testing.T) {
 		FeeProportionalMillionths: 190000,
 		FeeBaseMSat:               10,
 		TimeLockDelta:             40,
-		MessageFlags:              lnwire.ChanUpdateRequiredMaxHtlc,
+		HasMaxHTLC:                true,
 		MaxHTLC:                   4000,
 		MinHTLC:                   100,
 	}