1.0.0-alpha8: alpha7 field-test fixes

Two showstopper bugs surfaced by alpha7 field testing on a real
LSCache-fronted Drupal 11 site. Neither required architectural
rework; both are one-line fixes against the alpha7 substrate. No
contract changes, no new features.

Operators on alpha6 should upgrade past alpha7 directly to alpha8;
alpha7 should not be deployed.

- cwv_update_10003 split into addField + addIndex so the MySQL
  schema driver can normalise the index without looking up a
  column type that doesn't exist yet. Existing alpha6 sites can
  now upgrade.
- BeaconController async-context foreach iterator renamed from
  $value to $async_value to break the variable shadow with the
  float metric value declared earlier in receive(). Every alpha7
  beacon with at least one async observation was being silently
  dropped.

README documents the synthetic-probe accuracy caveat on
edge-cached and proxied stacks.