perf: add Rollup tree-shaking pre-pass for export * as barrels

Bun's bundler (and esbuild) cannot tree-shake `export * as X from "./mod"`
barrels — see evanw/esbuild#1420. Rollup can, using AST-level analysis to
drop unused exports and their transitive imports.

This adds a Rollup pre-pass to the build pipeline that runs before Bun's
compile step. Rollup resolves internal source, eliminates dead code across
the 57 `export * as` barrels, and writes tree-shaken ESM to a temp directory.
Bun then compiles the pre-processed output into the final binary.

- Add `script/treeshake-prepass.ts` with Bun.Transpiler-based Rollup plugin
- Integrate into `script/build.ts` (skippable via `--skip-treeshake`)
- Add `rollup` as devDependency
- Pre-pass completes in ~5s, negligible vs full multi-platform build

Current bundle savings are modest (~0.7%) because the single-entrypoint
architecture means most code paths are reachable. Savings scale with:
- Per-command entry points or lazy loading
- Remaining 50 `export namespace` → `export * as` migrations
- Future code splitting

https://claude.ai/code/session_01R7zMpXjsq1R6uR7xpyJ14i

bun.lock

@@ -450,6 +450,7 @@
         "@typescript/native-preview": "catalog:",
         "drizzle-kit": "catalog:",
         "drizzle-orm": "catalog:",
+        "rollup": "4.60.1",
         "typescript": "catalog:",
         "vscode-languageserver-types": "3.17.5",
         "why-is-node-running": "3.2.2",