more tweaks to hackathon project

test-server/configurator-extension/.gitignore

@@ -1,2 +1,2 @@
-# Copied in from packages/analytics-browser/lib/scripts after a build; see README.md.
+# Bundles and their .map files, copied in by sync-vendor.mjs after a build; see README.md.
 vendor/

test-server/configurator-extension/README.md

@@ -6,17 +6,26 @@ configurator can generate the code, but only an extension can put it on someone
 
 ## Setting it up
 
-The SDK bundles aren't checked in. Build them once, then vendor them in:
+It isn't in the Chrome Web Store, and a `.crx` can't be dragged into Chrome any more, so either route ends
+at Load unpacked on a folder.
+
+**From the test server.** `/configurator-extension.zip` is this directory, vendored bundles and all, zipped
+on request by `test-server/extension-archive.js`. Download it from the configurator's install panel or
+directly, unzip it, and load the `configurator-extension` folder it leaves behind. Nothing to build, and
+the archive always matches the checkout that served it. `vite build` emits the same file, so a hosted copy
+of the configurator offers the same download.
+
+**From this checkout.** The SDK bundles aren't checked in. Build them once, then vendor them in:
 
 ```bash
 pnpm --dir packages/analytics-browser build
 pnpm --dir packages/plugin-session-replay-browser build
 node test-server/configurator-extension/sync-vendor.mjs
 ```
 
-Load `test-server/configurator-extension` at `chrome://extensions` with developer mode on, and start the
-test server with `pnpm dev`. After editing any file here, hit the extension's reload icon — that also
-clears which tabs were being instrumented.
+Then load `test-server/configurator-extension` itself at `chrome://extensions` with developer mode on, and
+start the test server with `pnpm dev`. This is the one to use while working on the extension: after editing
+any file here, hit its reload icon — that also clears which tabs were being instrumented.
 
 ## Using it
 
@@ -25,6 +34,12 @@ to the "Run on URL" button, and click it. The page hands the configuration to th
 that URL in a new tab and initialises the SDK there. The note beside the button says what was installed,
 and the tab's console logs every event the SDK builds.
 
+Filling in "Mock Referrer" alongside that URL makes `document.referrer` read whatever you put there, so
+attribution can be tried without having to arrive from the referring site. "Clean Session", which is on
+unless you turn it off, deletes Amplitude's stored state for the site first, so every run starts with a new
+device ID, a new session and no prior campaign — which is what makes a referrer worth mocking in the first
+place. Untick it to pick up where the last run left off.
+
 **On the tab you're looking at.** Click the toolbar button to instrument the current tab with whatever the
 configurator sent last, or a debug-everything default if it hasn't sent anything yet. The badge reads `on`,
 and clicking again switches it off. Either way the tab reloads, since that's the only way to catch a page
@@ -34,9 +49,12 @@ from the start.
 
 `configurator-bridge.js` sits on the configurator page and relays `window.postMessage` requests to
 `background.js`. Its `matches` in the manifest cover the hosts the test server uses — `localhost` and
-`127.0.0.1` over http for `pnpm dev`, `local.website.com` over https for `pnpm dev:ssh` — and nothing
-else. Ports aren't part of a match pattern, so any port is covered, but serving the configurator from a
-host that isn't listed is why the page would report no extension. It's scoped to the configurator's own
+`127.0.0.1` over http for `pnpm dev`, `local.website.com` over https for `pnpm dev:ssh` — plus the Netlify
+site the `pnpm build:configurator` artifact is shared from, and nothing else. Ports aren't part of a match
+pattern, so any port is covered, but serving the configurator from a host that isn't listed is why the page
+would report no extension. Hosted origins are listed one at a time rather than as `https://*.netlify.app/`:
+a wildcard there would offer the relay to every site on a shared domain, and the relay leads to a service
+worker that can inject the SDK into any tab. It's scoped to the configurator's own
 path rather than a host wildcard so the relay isn't offered to every site, given how much the extension is
 allowed to do. Going through the page rather than `chrome.runtime.sendMessage` means the page needs no
 extension ID and no `externally_connectable` entry, and it lets the extension announce itself so the
@@ -83,6 +101,22 @@ page uses, and travels as JSON. Regexes have nowhere to live in JSON, so `toJson
   `base::IsStringUTF8`, which rejects Unicode non-characters, and the session replay bundle contains four
   literal U+FFFE characters. Chrome rejects the whole file with "It isn't UTF-8 encoded", which is why
   `sync-vendor.mjs` escapes them on the way in rather than a plain `cp`.
+- **Both landing options are spent on the first page of a run.** A mocked referrer and a cleared session
+  describe arriving at a site rather than being on one, so `takePayload()` hands them to the commit that
+  opens the run and takes them off the stored payload. Clearing on every commit would hand out a new device
+  ID and session on every page and no session would last more than one pageview; a referrer mocked again
+  would keep claiming the visitor came from elsewhere when they came from the previous page. Pages after
+  the first therefore report their real referrer and keep the session that was just started.
+- **A mocked referrer is only the JS view.** `handOver` shadows `document.referrer` with an own property,
+  which is what the campaign parser and the page-URL enrichment plugin read. The `Referer` header the page
+  was actually fetched with is untouched, so anything server-side still sees the truth — and modifying that
+  header wouldn't help, because Chrome derives `document.referrer` from the navigation's referrer rather
+  than from a header a `declarativeNetRequest` rule rewrote.
+- **Clearing the session takes every Amplitude key with it.** It sweeps by prefix — `AMP_` and the legacy
+  lowercase `amp_` — across cookies, `localStorage` and `sessionStorage`, so it also clears the state of the
+  site's *own* Amplitude instance if it has one, in your browser only. Nothing else the site stores is
+  touched, and IndexedDB is left alone: session replay's recorded events live there, and a new session ID
+  makes them moot anyway.
 - **Where events go.** The API key comes from the configurator, and events land in whatever project owns
   it. Use a scratch project, not a customer's production key.
 - **The page's own Amplitude.** The bundle merges itself onto an existing `window.amplitude`, which would