test(web): price the seeded compaction's shadow count exactly

The token-meter projections now subtract compact/summary's
shadowedTokenCount verbatim, so the seeded-history fixture's hand-picked
10k count drove the message figure negative and killed the projections
baseline. Realize the seed first (realizeSeedFixture, idempotent, shared
with seedSession) and derive the count from the shadowed rows through
the composed meter, reproducing the previous per-node fold bit for bit —
the recorded aria goldens pass unchanged.

apps/web/tests/scaffold.ts

@@ -476,15 +476,29 @@ export function fixtureUserPrompts(fixtureText: string): string[] {
  * @param id - the seeded session id (stable for deterministic goldens).
  * @returns the seeded id.
  */
-export async function seedSession(scaffold: WebScaffold, fixtureText: string, id: string): Promise<SessionId> {
+/**
+ * Realize a recorded seed fixture against one scaffold: substitute the
+ * `{{sessionId}}`/`{{cwd}}` placeholders and rewrite the recorded cwd to the
+ * scaffold's workspace. Idempotent, so a caller may realize early (e.g. to
+ * price content exactly as the host will fold it) and still pass the result
+ * through {@link seedSession}.
+ * @param scaffold - the booted scaffold whose workspace the seed targets.
+ * @param fixtureText - the committed seed fixture text.
+ * @param id - the session id the seed is realized for.
+ * @returns the realized fixture text.
+ */
+export function realizeSeedFixture(scaffold: WebScaffold, fixtureText: string, id: string): string {
   const realized = fixtureText
     .split('{{sessionId}}').join(id)
     .split('{{cwd}}').join(scaffold.workspaceCwd)
   const fixtureCwd = (JSON.parse(realized.split('\n', 1)[0]!) as { cwd?: string }).cwd
-  const rewritten = fixtureCwd === undefined
+  return fixtureCwd === undefined
     ? realized
     : realized.split(fixtureCwd).join(scaffold.workspaceCwd)
-  const events = parseSessionLog(rewritten)
+}
+
+export async function seedSession(scaffold: WebScaffold, fixtureText: string, id: string): Promise<SessionId> {
+  const events = parseSessionLog(realizeSeedFixture(scaffold, fixtureText, id))
   if (events.length === 0) throw new Error('seed fixture has no events')
   const last = events[events.length - 1]!
   // An open final turn would be mutated by resume's crash repair on first