Add priority-based impact reason ordering

Summary:
btd v1 checks change reasons in priority order (Inputs > Labels > Hash >
CiSrcs > Package > Rule) in a single pass, so the highest-priority reason
wins. btd v2 discovers changes in two phases — graph update finds
Labels/Hash/Rule first, then owners query finds Inputs later. With
first-writer-wins in record_impact, Hash was winning over Inputs.

This adds priority() to RootImpactKind and makes record_impact override
lower-priority reasons with higher-priority ones. Now when the owners query
finds a target already marked as Hash, it overrides with Inputs, matching
v1's attribution.

Also fixes deduplicate_target_categories to remove targets from
non_recursive_targets when they also appear in recursive_targets (e.g.
Labels overridden by Inputs changes non-recursive to recursive).

Reviewed By: aniketmathur

Differential Revision: D93274880

fbshipit-source-id: b0bb6a0f66b5c90fe62c4c519dd52a7ef01dafc1

Author: Aaron Ho

btd/src/diff.rs

@@ -248,6 +248,20 @@ impl RootImpactKind {
             | Self::ManualForRerun => true,
         }
     }
+
+    /// Higher value wins when multiple reasons apply to the same target.
+    pub const fn priority(self) -> u8 {
+        match self {
+            Self::New | Self::Remove | Self::ManualForRerun => 7,
+            Self::Inputs => 6,
+            Self::Labels => 5,
+            Self::Hash => 4,
+            Self::CiSrcs => 3,
+            Self::Package => 2,
+            Self::Rule => 1,
+            Self::PackageValues | Self::UniversalFile | Self::SelectAll => 0,
+        }
+    }
 }
 
 pub fn immediate_target_changes<'a>(