test(diff): automate DiffView/CodeBlock horizontal-scroll coverage (closes #21) (#101)

Turns the manual QA ask ("verify DiffView + CodeBlock horizontal-scroll
on-device with a populated diff") into two automated layers:

1. Unit (deterministic, runs in `npm test` now): extracted the shared
   ScrollView props into src/lib/scroll-config.ts (WIDE_CONTENT_SCROLL_CONFIG)
   so DiffView.tsx and CodeBlock.tsx spread the SAME plain object their tests
   assert on — no react-native-renderer needed. Added a source-scan
   regression test (wide-content-scroll.regression.test.ts) that fails if
   either component loses its ScrollView wiring or reintroduces
   numberOfLines truncation.

2. E2E (Maestro): .maestro/flows/diff-scroll.yaml opens a session with a
   pre-seeded wide edit-diff tool call and a wide fenced code block, then
   swipes each horizontal ScrollView left and asserts the off-screen marker
   text becomes visible. mock-opencode-server.ts gained a --seed-diff mode
   that serves this session via GET /session/:id/message (pre-existing
   history), not SSE — issue #90 (a separate SSE-render bug) is being fixed
   independently, and this flow must not depend on it landing first. Wired
   the new flow + port 4100 into run-e2e-flows.sh and activation-e2e.yml.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

scripts/run-e2e-flows.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 #
 # Runs the Maestro activation E2E flows against the mock opencode servers the
-# workflow started on the host (ports 4096-4099). Invoked as a single line from
+# workflow started on the host (ports 4096-4100). Invoked as a single line from
 # activation-e2e.yml's emulator-runner `script:` so cd/trap/loop actually work.
 #
 # Networking: `adb reverse` maps each mock port so the emulator's own
@@ -11,14 +11,14 @@ set -uo pipefail
 
 ROOT="$(pwd)"                       # capture BEFORE any cd, so diag paths are absolute
 APK="android/app/build/outputs/apk/release/app-release.apk"
-FLOWS=(activation-positive activation-negative-401 directory-picker all-sessions variant-picker)
+FLOWS=(activation-positive activation-negative-401 directory-picker all-sessions variant-picker diff-scroll)
 mkdir -p "$ROOT/artifacts/screenshots" "$ROOT/artifacts/diag"
 
 echo "== installing APK =="
 adb install "$APK"
 
 echo "== forwarding mock ports into the emulator (adb reverse) =="
-for p in 4096 4097 4098 4099; do adb reverse "tcp:$p" "tcp:$p"; done
+for p in 4096 4097 4098 4099 4100; do adb reverse "tcp:$p" "tcp:$p"; done
 adb reverse --list
 
 # Decisive probe: can the EMULATOR actually reach the host mock via 127.0.0.1?