feat(apply): cross-repo blast-radius in the plan preview

§7 M3 (part 2): planning a change to a GROUP node now reports the descendant
apps owned by OTHER projects that the change fans out to — the cross-repo
signal an operator needs before touching shared config.

- OwnershipPreview gains `blast_radius: Vec<ProjectImpact>`; `group_blast_radius`
  walks the group's subtree (recursive CTE) and groups descendant apps by their
  nearest-claimed-ancestor project, excluding the planning project. Ancestor
  resolution is memoized per group (one walk per distinct descendant group, not
  per app).
- `pic plan` renders `blast_radius` rows (mode-agnostic).
- the apply_ownership journey gains a plan-preview case pinning both the
  ownership annotation (claim / conflict) and the blast radius (two other
  projects' apps surfaced) end to end.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/picloud-cli/src/client.rs

@@ -1548,12 +1548,21 @@ pub struct ChangeDto {
 }
 
 /// §7 M3 plan preview: `action` ∈ claim/owned/conflict/unclaimed; `owner` is
-/// the current owner's slug (for owned/conflict).
+/// the current owner's slug (for owned/conflict); `blast_radius` (M3.2) lists
+/// other projects' apps a group change fans out to.
 #[derive(Debug, Deserialize)]
 pub struct OwnershipPreviewDto {
     pub action: String,
     #[serde(default)]
     pub owner: Option<String>,
+    #[serde(default)]
+    pub blast_radius: Vec<ProjectImpactDto>,
+}
+
+#[derive(Debug, Deserialize)]
+pub struct ProjectImpactDto {
+    pub project: String,
+    pub apps: u32,
 }
 
 /// Response of `POST .../tree/plan` (Phase 5): a per-node diff + one combined