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.

scripts/run-gates.ts

@@ -228,7 +228,10 @@ function ciPrimaryGates(): Gate[] {
     ...docSyncLeafGates(),
     pnpmScript('module-graph', 'verify-module-graph', { label: 'module graph' }),
     pnpmScript('knip', 'knip'),
-    pnpmScript('build', 'build'),
+    // typecheck and build now drive the same root solution graph; without the
+    // dependency two concurrent `tsc -b` runs race the same tsbuildinfo files.
+    // The tsc step is an incremental no-op after typecheck.
+    pnpmScript('build', 'build', { needs: ['typecheck'] }),
     pnpmScript('publint', 'publint', { needs: ['build'] }),
     pnpmScript('node-next-types', 'verify-node-next-types', {
       label: 'node-next types',