Reload only the rows that changed

Refreshing rebuilt the whole tree, and TreeView.reload() with no
argument drops the scroll position. Watching a job meant the view jumped
every fifteen seconds.

The provider now reconciles instead of replacing. update() rebuilds from
the store, matches rows to the existing nodes by identifier, copies the
rendered fields onto them, and reports the topmost rows that actually
changed; main.js reloads just those. Node identity survives the merge,
which is what makes a targeted reload possible. An open row that fetches
its own children has that cache dropped so it refetches — the running-job
path. A job finishing now reloads one run's row and nothing else.

Sections also each describe what they draw, so a section only rebuilds
when its own content moved: run activity no longer disturbs the pull
request list, and an idle tree is left alone entirely. This does not slow
anything down — a section showing a run in progress still updates on
every poll, and contributes a time bucket so its elapsed clock keeps
moving.

A row set changing at the root still needs a whole-tree reload; there the
selected row is re-revealed afterwards, which is the closest thing to a
scroll anchor Nova exposes.

Adds seventeen checks: no-op updates reporting nothing, a finishing job
naming only its own row, node identity surviving, a new run reloading
just its workflow group, and a new workflow escalating to a full reload.
168 checks total.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01MQuusXgZC2dzwpJJ1qhtti

Tests/nova-stub.js

@@ -70,6 +70,11 @@ global.TreeView = class TreeView {
         treeViews.push(this);
     }
     reload() { this.reloads += 1; return Promise.resolve(); }
+    reveal(element, options) {
+        this.revealed = { element, options };
+        if (options && options.select) this.selection = [element];
+        return Promise.resolve();
+    }
     onDidExpandElement(cb) { this._onExpand.push(cb); return { dispose() {} }; }
     onDidCollapseElement(cb) { this._onCollapse.push(cb); return { dispose() {} }; }
     // Test helpers: pretend the user opened or closed a row.