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/verify-cordis-config.ts

@@ -152,12 +152,13 @@ function localPackageDirectories(): Map<string, string> {
 }
 
 function rootProjectReferences(): Set<string> {
-  // Typecheck runs two sibling aggregates (root = host program,
-  // tsconfig.client.json = client program; the two sides merge cordis Context
-  // under the same keys, so one program cannot see both). Seed both and follow
-  // any nested aggregate references to collect the covered leaf project set.
+  // The root solution references the host and client aggregates (the two
+  // sides merge cordis Context under the same keys, so one program cannot see
+  // both — but this BFS only collects reference paths, it never forms a
+  // program). Seed the solution and follow nested aggregate references to
+  // collect the covered leaf project set.
   const collected = new Set<string>()
-  const queue = [resolve(root, 'tsconfig.json'), resolve(root, 'tsconfig.client.json')]
+  const queue = [resolve(root, 'tsconfig.json')]
   const seen = new Set<string>()
   for (let file = queue.pop(); file !== undefined; file = queue.pop()) {
     if (seen.has(file)) continue