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

src/popup/views/addWallet.js

@@ -179,7 +179,7 @@ async function importMnemonic(ctx) {
 
     // Scan for used HD addresses beyond index 0.
     showFlash("Scanning for addresses...", 30000);
-    const scan = await scanForAddresses(xpub, state.rpcUrl);
+    const scan = await scanForAddresses(xpub, state.rpcUrl, state.networkId);
     if (scan.addresses.length > 1) {
         wallet.addresses = scan.addresses.map((a) => ({
             address: a.address,
@@ -298,7 +298,7 @@ async function importXprvKey(ctx) {
 
     // Scan for used HD addresses beyond index 0.
     showFlash("Scanning for addresses...", 30000);
-    const scan = await scanForAddresses(xpub, state.rpcUrl);
+    const scan = await scanForAddresses(xpub, state.rpcUrl, state.networkId);
     if (scan.addresses.length > 1) {
         wallet.addresses = scan.addresses.map((a) => ({
             address: a.address,