Single-source the version from package.json (closes #5)

src/index.ts imports package.json for VERSION and bin/quak.ts passes
VERSION to commander, so package.json is the only place the version is
written. tsc copies package.json to dist/package.json, so the import
resolves from the built output; script/build runs the built CLI with
--version to prove it. A test checks VERSION and quak --version against
package.json.

Model: opus-5-5

TODO.md

@@ -18,6 +18,12 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-23: Single-sourced the version string (issue 5). `package.json` is the
+  only place it is written: `src/index.ts` imports it for `VERSION` and
+  `bin/quak.ts` passes `VERSION` to commander. tsc copies `package.json` to
+  `dist/package.json`, so the import resolves from the built output too, and
+  `script/build` runs the built CLI with `--version` to prove it. A test checks
+  that `VERSION` and `quak --version` both equal the `package.json` version.
 - 2026-09-23: Tested that `Library.close()` waits for the originals precache
   (issue 93). The test that holds a precache fetch open while `close()` runs now
   runs once with only the thumbnail fill and once with only the originals fill,