release: package the extension, pin the Chrome extension id, and prove the wallet survives a reinstall (closes #310)

There was no packaging target anywhere, no artifact, and no `key` in
`manifest/chrome.json` — so an unpacked Chrome load derived its extension
id, and therefore its `chrome.storage.local` partition, from the absolute
checkout path. Moving or re-cloning the checkout presented an empty
wallet, with no error and nothing in the UI to say so.

`manifest/chrome.json` now carries a fixed `key`: the public half of an
RSA keypair, which pins the extension id to
`gipbhkogfopeahplcjhipkgpcimdpkip`. The private half is a credential and
is not in this repo; no target generates one into the working tree, and
`tests/extensionId.test.js` fails if a `.pem` is ever committed. Changing
`key` changes the id and orphans every wallet stored under the old one.

`make package` (script/package) runs `make build` — the only audited path
to a release build — and writes one self-contained, versioned archive per
browser into `release/`, plus `SHA256SUMS`. The archives are deterministic:
entries sorted, timestamps fixed, compression level fixed, so two builds
of one commit are byte-identical. Self-containment is checked rather than
assumed: every path the manifests and the popup HTML reference is resolved
and required to be inside the archive, a reference that climbs out of the
extension root is a hard failure, and files left at the `dist/` root —
`dist/styles.css`, which build.js copies into each browser directory — are
reported as deliberately not shipped rather than dropped by a glob. The
archive is then read back off disk and compared member by member against
the directory it was built from. The zip writer and reader are stdlib zlib
in `script/lib/zip.js`; no new dependency, and nothing unpinned.

One version, enforced rather than generated. `script/lib/version.js`
requires `package.json`, `manifest/chrome.json` and `manifest/firefox.json`
to agree and fails the build naming each file and what it said, instead of
reading from one of the three. `BUILD_COMMIT` now carries `-dirty` when the
working tree does not match `HEAD`, and `-unknown` when git cannot say;
the full hash behind the About screen's commit link stays clean so the link
still resolves.

Two real-browser observations, both run through the pinned harnesses:

- `tests/e2e/storagePartition.js` loads the build from two different paths
  in one Chrome profile. With `key`: same id, and the second load reads the
  first load's storage. Without `key`: different ids, and the second load
  sees an empty partition. Loading both keyed copies at once yields one id,
  not two.
- `tests/e2e/firefox/reinstall.js` installs the packaged XPI in a real
  Firefox, creates a wallet, quits the browser, restarts on the same
  profile, adds the add-on again, and decrypts the vault back to the
  original recovery phrase. It then observes that an explicit uninstall
  DESTROYS that storage — correct browser behaviour, but for a wallet it
  means Remove is irreversible except from the recovery phrase, so
  README.md says so.

Firefox ships an UNSIGNED XPI. README.md states plainly that release
Firefox and ESR will refuse it, that Developer Edition, Nightly or an
Unbranded build is required, and that a temporary add-on does not survive
a browser restart. AMO signing, CRX packing, tagging and any upload are
deliberately out of scope.

tests/e2e/firefox/driver.js

@@ -103,7 +103,13 @@ class Driver {
 
     // ------------------------------------------------------------ setup
 
-    async newSession() {
+    // `profileDir` reuses an existing profile directory in place instead of
+    // letting geckodriver make a throwaway one. That is the only way to ask
+    // what survives a browser RESTART, which for a Firefox add-on that can
+    // only be installed temporarily is the question that decides whether the
+    // extension is usable at all: a temporary add-on is unloaded when Firefox
+    // exits, so every session begins by adding it again.
+    async newSession(profileDir) {
         const prefs = {
             // See EXTENSION_UUID above. The pref is a string pref whose
             // value is itself JSON.
@@ -132,26 +138,27 @@ class Driver {
             "extensions.openPopupWithoutUserGesture.enabled": false,
         };
 
+        const args = [
+            "-headless",
+            // Mandatory on Firefox 153: without it, navigating to
+            // moz-extension:// and running chrome-context script both
+            // fail with "unsupported operation".
+            //
+            // It grants the driver FULL CHROME PRIVILEGES over this
+            // browser. Acceptable only because the browser is a
+            // throwaway in a CI container; never point a session with
+            // this flag at anything you care about.
+            "-remote-allow-system-access",
+        ];
+        if (profileDir) args.push("-profile", profileDir);
+
         const value = await this.send("POST", "/session", {
             capabilities: {
                 alwaysMatch: {
                     browserName: "firefox",
                     "moz:firefoxOptions": {
                         binary: FIREFOX_BIN,
-                        args: [
-                            "-headless",
-                            // Mandatory on Firefox 153: without it,
-                            // navigating to moz-extension:// and running
-                            // chrome-context script both fail with
-                            // "unsupported operation".
-                            //
-                            // It grants the driver FULL CHROME PRIVILEGES
-                            // over this browser. Acceptable only because
-                            // the browser is a throwaway in a CI
-                            // container; never point a session with this
-                            // flag at anything you care about.
-                            "-remote-allow-system-access",
-                        ],
+                        args,
                         prefs,
                     },
                 },
@@ -162,16 +169,30 @@ class Driver {
         return value;
     }
 
-    // Installs the unpacked MV2 build straight from a directory.
-    // temporary:true bypasses signature checks, so no XPI and no signing
-    // are involved, and the add-on dies with the profile.
-    async installAddon(dir) {
+    // Installs the MV2 build, either from an unpacked directory or from an
+    // XPI file. temporary:true bypasses signature checks — which is the only
+    // way an UNSIGNED xpi installs at all, and the reason README.md says
+    // release Firefox will refuse the artifact this repo produces — and the
+    // add-on dies with the profile.
+    //
+    // Returns the add-on id Firefox assigned, which is
+    // browser_specific_settings.gecko.id from the manifest and is what
+    // uninstallAddon() takes.
+    async installAddon(pathToAddon) {
         return this.session("POST", "/moz/addon/install", {
-            path: dir,
+            path: pathToAddon,
             temporary: true,
         });
     }
 
+    // Removes an installed add-on, the way clicking Remove in about:addons
+    // does. tests/e2e/firefox/reinstall.js uses it to ask the one question
+    // that decides whether this extension can be used at all on Firefox: does
+    // the vault survive being removed and added again.
+    async uninstallAddon(id) {
+        return this.session("POST", "/moz/addon/uninstall", { id });
+    }
+
     // Classic navigation on purpose. BiDi's browsingContext.navigate
     // refuses moz-extension:// URLs outright.
     async navigate(url) {