fix: run libsodium on WebAssembly under the extension CSP (closes #182)

libsodium ships a WASM build and a wasm2js translation in one file, tries
WASM first, and silently falls back if instantiation throws. Under a plain
script-src 'self' the fallback was taken on every popup load, announced by
nothing but an uncaught CompileError.

Measured on the vault's own Argon2id parameters (OPSLIMIT_INTERACTIVE,
MEMLIMIT_INTERACTIVE), node 22: WASM 141-198ms per derivation, wasm2js
3204-3660ms. The work factor is identical either way — it is set by the
ops and memory parameters, not by wall time — so the fallback bought no
security and cost about 3.5s on every operation that asks for the
password, which is every signature.

Both manifests now declare script-src 'self' 'wasm-unsafe-eval';
object-src 'self' for extension pages: an object under
content_security_policy.extension_pages for Chrome MV3, a bare string for
Firefox MV2. The keyword permits compiling WebAssembly and nothing else —
not eval() of strings, not inline script, not remote script — and reaching
it requires already executing script in an extension page. 'unsafe-eval'
is not granted.

The silence is what made this dangerous, so the fallback is now loud at
three levels: tests/manifest.test.js pins both policies to exactly that
token set, failing make check if the grant is dropped or if anything is
added beside it; tests/vaultBackend.test.js asserts the unit tests
exercise the WASM backend, with a self-validating check that libsodium
never swapped its fallback in; and the e2e suite compiles a WebAssembly
module inside the real popup under the real manifest, with the harness
allowlist entry that used to excuse the CompileError now deleted.

The runtime fallback itself is kept — a wallet that refuses to decrypt is
worse than a slow one — but vault.js now reports the backend and logs an
error when it is not WASM.

README.md

@@ -123,16 +123,17 @@ unavailable). The suite lives in `tests/e2e/` and is driven by
 `playwright-core`, whose version must stay matched to the container's Playwright
 version — the browsers ship inside the image.
 
-It covers popup load, wallet creation through the UI, the Add Token screen, the
-transaction detail screen for an ERC-20 transfer, and the recovery phrase screen
-— which wallet types are offered it, that it holds nothing before the password
-is accepted, that a wrong password reveals nothing, that leaving it by either
-route wipes it — including a leave taken while the decrypt is still running —
-and that reopening the popup does not land on it. All outbound network is
-intercepted at the browser level and served from fixtures in
-`tests/e2e/network.js`, so the run is deterministic and fully offline;
-unrecognised outbound requests are reported as failures rather than silently
-allowed.
+It covers popup load, WebAssembly compilation under the shipped CSP (see
+[Content Security Policy](#content-security-policy)), wallet creation through
+the UI, the Add Token screen, the transaction detail screen for an ERC-20
+transfer, and the recovery phrase screen — which wallet types are offered it,
+that it holds nothing before the password is accepted, that a wrong password
+reveals nothing, that leaving it by either route wipes it — including a leave
+taken while the decrypt is still running — and that reopening the popup does not
+land on it. All outbound network is intercepted at the browser level and served
+from fixtures in `tests/e2e/network.js`, so the run is deterministic and fully
+offline; unrecognised outbound requests are reported as failures rather than
+silently allowed.
 
 That reporting has one bound worth knowing. Observation ends when the browser
 context is torn down, and nothing can watch traffic after that, so the run keeps
@@ -1068,6 +1069,36 @@ battle-tested.
 Exceptions require explicit authorization in a code comment referencing this
 policy, but as of now there are none.
 
+### Content Security Policy
+
+Both manifests declare the same policy for extension pages —
+`script-src 'self' 'wasm-unsafe-eval'; object-src 'self'` — as an object under
+`content_security_policy.extension_pages` in `manifest/chrome.json` (MV3) and as
+a bare string in `manifest/firefox.json` (MV2).
+
+`'wasm-unsafe-eval'` is there for one reason: libsodium. It ships a WebAssembly
+build and a `wasm2js` translation of it in one file, tries WASM first, and
+silently falls back to the translation if instantiation throws. Under a plain
+`script-src 'self'` the fallback was taken on every popup load, announced by
+nothing but an uncaught `CompileError`. Measured on the same Argon2id parameters
+the vault uses (`OPSLIMIT_INTERACTIVE`, `MEMLIMIT_INTERACTIVE`), WASM derives a
+key in 141-198ms and `wasm2js` in 3204-3660ms. The work factor is identical — it
+is set by the ops and memory parameters, not by wall time — so the fallback
+bought nothing and cost about three and a half seconds on every operation that
+asks for the password, which is every signature.
+
+The keyword permits compiling WebAssembly and nothing else: not `eval()` of
+strings, not inline script, not remote script. Using it requires already
+executing script in an extension page, which is complete compromise on its own.
+`'unsafe-eval'` is a different proposition and is not granted.
+
+The grant is pinned in both directions. `tests/manifest.test.js` asserts the
+exact token set in both manifests, so dropping `'wasm-unsafe-eval'` (a silent
+20x regression on the key derivation) and adding anything beyond it both fail
+`make check`. `tests/vaultBackend.test.js` asserts the unit tests run the WASM
+backend, and `make test-e2e` compiles a WebAssembly module inside the real popup
+under the real manifest.
+
 ### DEBUG Mode Policy
 
 The `DEBUG` constant in the popup JS enables a red "DEBUG / INSECURE" banner and