Adds the Mermaid configuration renderer

[johnnyt]

Why

The Livebook inspector (sui-t36) needs its first configuration rendering:
a way to show a chart's structure with the active states highlighted,
consuming only the compiled machine and a configuration. The bead settles
the research-doc open question by starting with Mermaid and accepting its
documented limit, keeping the interface stable for the later elkjs swap
(ADR-0008 fixes the destination stack; this is the epic's permitted first
cut).

What

StatifierUI.Diagram.render/2 - a pure function from Statifier.Machine
plus an active configuration (any enumerable of state indexes; the full
configuration per ADR-0005) to Mermaid stateDiagram-v2 source suitable
for Kino.Mermaid. Compound states render as composite blocks, parallel
states as composites with -- region dividers, resolved initial states as
[*] markers, final and history states with label suffixes, and active
states via a classDef/class pair. Nodes are aliased s<index> from the
engine's document-order state indexes, so consumers address nodes by the
identity vocabulary of statifier ADR-0012.

Mermaid cannot draw a transition between internal states of two different
composite states, so such edges are lifted to the composite siblings under
the least common ancestor with a [lifted: source -> target] marker; deep
initial targets get a [deep: ...] marker the same way. The compromise is
documented in the moduledoc.

Notes

• Pure module: no Kino (or LiveView) dependency; the widget assembly bead
  (sui-t36.8) is where Kino.Mermaid consumes this.
• Does not touch the trace wire format or fixtures contract; if a later
  slice wants live re-rendering from effect.datamodel_change, that rides
  on Serializes datamodel_change as a wire type #27 rather than anything here.
• Full mix quality green locally (255 tests, 95.3% coverage); mix gate.verify attests the run was not narrowed; mix gate.check reports
  no unjustified gate changes. Gettext/Sobelow are the two permanently
  inapplicable skips.

Closes: sui-t36.4