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

TODO.md

@@ -45,6 +45,50 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-08-23: The background no longer reads or writes the shared `state`
+  singleton ([#324](https://git.eeqj.de/sneak/AutistMask/issues/324)), which
+  also closes the cold-worker wrong-chain send
+  ([#320](https://git.eeqj.de/sneak/AutistMask/issues/320)). One in-memory copy
+  loaded once is the popup's lifetime, not the MV3 worker's: the worker is
+  killed when idle, nothing loaded state at module scope, and an unpopulated
+  read was answered out of `DEFAULT_STATE` in silence. Five defects traced to
+  that, and every point fix added a `loadState()` that created the next one — a
+  load detaches the objects an in-flight handler is holding. The background now
+  has its own storage layer (`src/background/state.js`): `getState()` for a
+  detached per-call read, `updateState()` for a queued read-modify-write.
+  `backgroundRefresh()` refreshes a private copy and applies the balances that
+  came back by address, so a wallet added, renamed or deleted during the round
+  trip survives. The transaction attempt takes its chain id and its endpoint
+  from one snapshot, so a committed chain switch can no longer move the endpoint
+  under an artifact already verified against the old chain. `getProvider()` now
+  REQUIRES the network id, which is what closes
+  [#320](https://git.eeqj.de/sneak/AutistMask/issues/320) at the shape rather
+  than at the call site. The prohibition is enforced by `build.js`, which fails
+  the build when esbuild's own metafile reports `src/shared/state.js` as an
+  input of either background bundle — the resolution the shipped bundle was
+  actually built from, so no specifier syntax and no resolution rule can slip
+  past it, and `make build` runs in CI. A bundled entry point under
+  `src/background/` with no line in the table fails the build too, so a second
+  worker entry point is protected by default rather than only if whoever adds it
+  knows the table exists. The assertion itself is pinned by
+  `tests/buildForbiddenInputs.test.js`, including every way its table can rot: a
+  key no bundled entry point matched, a forbidden module this build bundled
+  nowhere, and an entry that lists no modules (which would otherwise empty the
+  lint rule's forbidden set as well, and is refused at require time). Its bound
+  is that it is keyed by path, so a COPY of the singleton at another path is
+  outside it — loud for three of the five defects and silent for the other two;
+  the bounds are recorded in full where the table lives
+  (`script/lib/forbiddenBundleInputs.js`). An ESLint rule that walks the require
+  graph textually gives the same answer in the editor, before a full bundle; it
+  reads the same table, and it is fast feedback rather than the guarantee. The
+  shapes it catches are pinned by `tests/backgroundStateLintRule.test.js`, and
+  so are the two it misses — a computed specifier and a symlink — as asserted
+  non-reports, which the build fails on. Reading an unloaded singleton now
+  throws `StateNotLoadedError` instead of serving defaults. The
+  `chrome.storage.local` stubs in eight test files aliased instead of
+  structured-cloning, which could let an assertion pass on a build that never
+  wrote anything; every test that drives real persistence now goes through
+  `tests/support/storageStub.js`.
 - 2026-08-23: A swap always names its output token
   ([#346](https://git.eeqj.de/sneak/AutistMask/issues/346)). The `Token Out`
   detail line in `src/shared/uniswap.js` was pushed only when a symbol was