refactor(tsconfig): single root solution graph over host/client aggregates

Root tsconfig.json becomes a pure solution (files:[] + two references);
the former root host aggregate moves verbatim to tsconfig.host.json.
New tsconfig.base.client.json carries the shared client compiler shape
(jsx/DOM lib/types:[]), and tsconfig.client.json plus the 12
packages/client tsconfigs extend it instead of restating the trio.
tsconfig.build.json is deleted: the solution covers the full emit graph,
absorbing the command-goal typecheck/build drift.

Consumers migrate to the single graph: typecheck/build scripts and the
lefthook pre-push hook run bare `tsc -b` (pre-push now covers the client
side); the run-gates build gate needs typecheck so two concurrent tsc -b
runs cannot race the same tsbuildinfo; ts-project.ts and the standalone
doc-typecheck mode seed tsconfig.host.json explicitly (never the root
solution — flattening host+client into one program collides the cordis
Context merges); verify-cordis-config BFS seeds the root solution alone.

Per missions/tsconfig-single-graph-migration.md §2–§3.

tsconfig.base.client.json

@@ -0,0 +1,11 @@
+{
+  // Client-side compiler shape shared by tsconfig.client.json and every
+  // packages/client/* package: browser lib surface, React JSX, no ambient
+  // node types (packages that need them override locally).
+  "extends": "./tsconfig.base.json",
+  "compilerOptions": {
+    "jsx": "react-jsx",
+    "lib": ["ES2024", "DOM", "DOM.Iterable"],
+    "types": []
+  }
+}