Add D2 output format

[KTSCode]

Add D2 output format

Adds D2 as a rich output format for mix ecto.gen.erd,
alongside DOT, PlantUML, DBML, QuickDBD, and Mermaid.

D2 is ER-native and renders to SVG without external runtime dependencies, so it
fills the gap between DOT/PlantUML (rich, but need Graphviz/Java) and
Mermaid/DBML (portable, but table-only).

What it renders

Everything DOT and PlantUML can show, plus more:

• Schemas, embedded schemas, and m2m-derived tables via D2's sql_table shape
• Crow's-foot cardinality on both ends of a <-> connection, the primary-key side is cf-one-required (exactly one), the foreign-key side cf-many-required (or cf-one for has-one) ER-native, like PlantUML/Mermaid
• Primary and foreign keys as native constraint: badges, FK marking exceeds DOT, which marks none
• Clusters as colored D2 containers (set via :map_node + Ecto.ERD.Node.set_cluster/2)
• Elixir-style types — #Enum<[...]>, #Ecto.Embedded<...>, {:array, _} (shares the DOT renderer's type formatter, so :enum_values_limit / :enum_values_order work identically)

Example diagram (source):

Usage

$ mix ecto.gen.erd --output-path=ecto_erd.d2
$ mix ecto.gen.erd --output-path=ecto_erd.d2 && d2 ecto_erd.d2 erd.svg

Design

The guiding principle: mirror DOT (the library's reference rich format), and
deviate toward another format's behavior only where it yields more or better
information.

• Cardinality crow's-foot on both ends, not DOT's plain edges. D2 is
  ER-native; DOT's plainness is a Graphviz limitation, not a style to copy.
• Foreign keys marked natively. DOT omits FK marking because its HTML
  tables have no clean affordance; D2's sql_table does, so it should.
• :columns accepts [] (bare nodes) or the default [:name, :type],
  raising otherwise. Type-only or reordered subsets are impossible on
  sql_table (rows are fixed name/type pairs); name-only bare rows would
  compile in d2 but stay unsupported for parity with Mermaid. With [], edges
  collapse to node level and are deduped per node pair.
• Collision-safe keys container keys start with cluster_<name> and
  escalate the prefix when that would collide with a global node. Because D2
  folds keys case-insensitively, case-only node or cluster collisions receive
  distinct internal keys while retaining their original displayed names via
  label:.
• :fontname ignored; D2 has no in-source font-family control (as with
  DBML/QuickDBD/Mermaid).

Notes

• Example images are SVG, not PNG. D2's PNG export downloads a ~140 MB
  headless Chromium; SVG is native and dependency-free.
• Arrowheads: d2 v0.7.1 silently ignores source-arrowhead on a ->
  connection but renders both ends on <-> (verified empirically), so edges
  use the bidirectional operator with both ends marked.
• Equal field names and types: d2 suppresses a sql_table type cell when
  its text equals the row key, such as an :id field of type :id. The
  renderer adds an invisible trailing space to the type text so the value
  remains visible without changing the field key used by edge ports.
• Escaping: quoted D2 strings escape \, ", ${ (variable
  substitution), and newlines, so hostile schema/source names can't break the
  document or trigger d2 variable interpolation.

Testing

• Unit tests cover type formatting (enums, embeds, arrays), PK/FK/combined
  constraints, both-end crow's-foot cardinality, clusters, cluster/node key
  collisions (including case-only node and cluster identities), equal
  field-name/type rendering, string escaping, node-pair edge dedup with
  columns: [], the :columns fallback, and embedded-schema edges.
• Rich and columns: [] collision cases compile cleanly with d2 v0.7.1
  while preserving distinct displayed nodes and containers.
• Validated end-to-end against a production Phoenix app (51 schemas, 89
  relationships, 16 enums, 11 embeds) all rendered, zero schemas skipped, and
  the output compiled to SVG.

[fuelen]

Hi Kyle!
Thank you for the contribution! I'll review it later once I take my personal laptop back from the service center 🙂

[fuelen]

Great work! Please remove the asset, update the README accordingly, and we're good to go.

P.S: don't worry about examples/d2/ being empty. I see that the examples generator is broken for every format at the moment, unrelated to your PR.

[fuelen]

Could you please remove this demo file? I understand the point, and nothing is wrong with the image. DOT is the de facto default here and the README carries a single demo. How to present several formats is a separate question I want to think through on its own rather than settle in this PR.