feat(diagram-engine): style markers + XML→tree reverse parser

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Groundwork for a declarative diagram engine where the model declares nesting and
the engine computes every coordinate, instead of the model emitting raw mxCell XML.

The design keeps the canvas as the SINGLE source of truth: the node tree is never
persisted, it is re-derived from the current canvas XML whenever needed. A user's
manual edits are therefore an input to the next re-layout, not a second copy of
the state that has to be reconciled.

Two behaviours this relies on, both verified against the real embedded editor with
a Playwright mouse drag (reading the editor's own autosave payload):

  1. An AWS group stencil WITHOUT container=1 does not get a dragged shape
     reparented — parent stays "1" and geometry stays absolute. WITH container=1
     it does: parent becomes the frame, geometry becomes parent-relative. So the
     engine must stamp container=1 on every container it emits.
  2. draw.io preserves style keys it does not understand, and resolves a duplicate
     key last-wins. So dai_* markers survive a user edit, and container=1 can be
     appended to a catalog style without first parsing out an existing value —
     which matters because the AWS catalog is inconsistent about it (group_vpc,
     group_region, group_subnet ship without it; group_account ships with it).

markers.ts  — dai_kind / dai_dir / dai_gap / dai_cols / dai_pin, and the container
              token normalisation.
types.ts    — the node tree contract, plus a `foreign` bucket so cells the engine
              does not understand (user annotations, imported shapes) round-trip
              verbatim rather than being destroyed by a re-layout.
parse.ts    — XML → tree. Handles all four icon encodings (resIcon=, bare shape=,
              shape=image data URI, grIcon=), resolves nesting from parent with a
              geometry fallback for frames that lack container=1, recovers layout
              direction from the marker or infers it from child positions, and
              survives cycles, compressed files and multi-page decks.

75 unit tests, including a round-trip against real output from the reference
project's build_vpc.mjs.

One finding worth recording: the reference project's "phantom" node (a wrapper that
participates in layout but emits no cell) makes the round-trip lossy by
construction. In build_vpc.mjs a phantom erased a container's col direction — its
children were reparented onto the grandparent, leaving a 2-D arrangement the parser
can only read as a grid. 26 of the reference project's 31 examples use phantoms, so
our engine needs an invisible-but-real container instead. Tracked separately.

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This commit is contained in:

dayuan.jiang

2026-08-09 11:35:32 +09:00

parent fd758b9e87

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