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.

package.json

@@ -14,10 +14,10 @@
     "website"
   ],
   "scripts": {
-    "build": "tsc -b tsconfig.build.json && tsdown",
+    "build": "tsc -b && tsdown",
     "build:web": "pnpm --filter @deepseek-ai/dsh-frontend run build",
     "clean:build": "rm -rf .typecheck packages/*/*/lib vendor/*/lib *.tsbuildinfo",
-    "typecheck": "tsc -b tsconfig.json tsconfig.client.json",
+    "typecheck": "tsc -b",
     "lint": "eslint .",
     "lint:fix": "eslint . --fix",
     "duplication": "jscpd --config .jscpd.json packages scripts",