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/ts-project.ts

@@ -22,9 +22,13 @@ const configHost: ts.ParseConfigFileHost = {
   },
 }
 
-/** Parse a root tsconfig and flatten all referenced projects into one semantic graph. */
+/**
+ * Parse the host aggregate tsconfig and flatten all referenced projects into one
+ * semantic graph. Never seed the root solution: flattening host+client into one
+ * program collides the cordis Context merges.
+ */
 function loadProjectGraph(projectRoot: string): ProjectGraph {
-  const rootConfigPath = resolve(projectRoot, 'tsconfig.json')
+  const rootConfigPath = resolve(projectRoot, 'tsconfig.host.json')
   const rootConfig = parseConfig(rootConfigPath)
   const rootNames = new Set<string>()
   const visited = new Set<string>()