Serializes datamodel_change as a wire type

[johnnyt]

Why

Since sui-bpb landed session.datamodel, a wire consumer can see the
datamodel's variable names but never a value: the snapshot is taken before
the binding fold, so every <data> element arrives as {"$undefined": true},
and the DatamodelChange effects that carry the actual writes were dropped
with an {:unknown_effect, :datamodel_change} warning per assignment. This
serializes them, closing the gap and unblocking the datamodel explorer pane
(sui-t36.7).

What

• New wire type effect.datamodel_change (the tenth effect.* type), one
  message per successful datamodel write, consuming statifier's
  Statifier.Effect.DatamodelChange (st-oef3) as-is - no upstream change.
• location_path serializes as a heterogeneous JSON array: string segments
  are object keys, integer segments are 0-based array indexes. JSON's own
  typing carries the distinction, so no tagging is needed; location_source
  keeps the raw author string alongside.
• new_value/prior_value follow the _event.data three-way absence rule
  (:undefined omits the key, nil is JSON null), because ADR-0037's
  unbound sentinel makes nil a genuinely stored null here. The normalizer's
  put_event_data/2 generalizes to put_defined/3 for all three fields.
• The owner vocabulary gains the widened "invoke" kind for
  <invoke idlocation> writes (Owners table grows to six variants).
• docs/wire-format.md: full payload schema, the location_path and
  mutual-exclusivity (d_index/c_index) rules, type index row 24, and the
  versioning decision recorded explicitly - the format version stays 1,
  since adding a type is additive under the must-ignore rule.
• Tests: @coverage grows to 19 pairs; a new describe covers the assign,
  <data>-binding, and <invoke idlocation> shapes plus the value codec;
  the type-index drift test now checks 24 types.

Notes

• Full mix quality green (243 tests, 95.4% coverage, dialyzer clean; the
  two permanent skips, Gettext and Sobelow, are declared not-applicable).
  mix gate.verify also run directly and green.
• The @coverage maximal literal populates d_index and c_index together,
  a shape the engine never emits - deliberate, so the produced key set covers
  the full documented schema; the real mutually-exclusive shapes have their
  own tests. Called out in a comment at the table.

Closes sui-h92