harden: stop the background reading the shared state singleton, and enforce it at build time (closes #324)

Five defects, one of which destroyed every wallet, came from src/background reading and writing the module-level state singleton the MV3 worker never populates, which silently served DEFAULT_STATE. Each point fix created the next defect. The background now has its own per-call getState() and a queued read-modify-write updateState(); the singleton is unreachable from it, and an unpopulated read throws instead of serving defaults.

The prohibition is enforced by the build, not by review: build.js asserts over esbuild's own metafile that no forbidden module is an input of a background bundle, so every specifier syntax esbuild resolves is covered, and both halves of the table are checked for rot -- a stale key, a stale module, an empty list, or an unlisted entry point under src/background/ all fail the build. The ESLint rule remains as fast local feedback and reads the same shared table. Known bounds are documented where the table lives.

Also closes #320: getProvider() now requires a validated network id, so a cold worker no longer prepares a non-mainnet dApp transaction for mainnet and gets refused by the wallet's own verifier. backgroundRefresh() no longer mutates address objects across a network round trip, the broadcast path takes its endpoint and chain id from one snapshot, and eight test storage stubs now structured-clone on get as the real chrome.storage.local does.

closes #320

tests/txStatus.test.js

@@ -96,18 +96,14 @@ global.document = {
 
 global.window = { location: { search: "" } };
 
-const stored = {};
-global.chrome = {
-    storage: {
-        local: {
-            set: (obj) => {
-                Object.assign(stored, obj);
-                return Promise.resolve();
-            },
-            get: () => Promise.resolve(stored),
-        },
-    },
-};
+// Clones in both directions, as the real chrome.storage.local does; the stub
+// here used to hand back the live stored object, so an in-memory mutation
+// looked like a write that had reached storage. See
+// tests/support/storageStub.js.
+const { makeStorageStub } = require("./support/storageStub");
+
+const storage = makeStorageStub();
+global.chrome = { storage };
 
 const txStatus = require("../src/popup/views/txStatus");
 const { state } = require("../src/shared/state");