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.

tsdown.config.ts

@@ -2,7 +2,7 @@ import { defineConfig } from 'tsdown'
 
 /**
  * JS bundling for vendored Cordis and Harness TypeScript packages.
- * TypeScript source is compiled first by `tsc -b tsconfig.build.json`; tsdown
+ * TypeScript source is compiled first by `tsc -b` (the root solution); tsdown
  * reads only the emitted JS under lib/types and writes the package root and
  * invariant companion runtime bundles. Declarations are NOT produced here,
  * hence `dts: false`.