fix(flowchart): preserve raw subgraph direction value; normalize TD→TB only for dagre

[Copilot]

📑 Summary

addSubGraph() was normalizing direction TD to TB before storing it on the subgraph object. This caused subgraph.dir to return 'TB' instead of the user-authored 'TD', breaking the assertion:

packages/mermaid/src/diagrams/flowchart/parser/subgraph.spec.js
> when parsing subgraphs > should correctly parse direction TD inside a subgraph
AssertionError: expected 'TB' to be 'TD'

📏 Design Decisions

• addSubGraph(): removed the TD→TB normalization from the storage path. subGraph.dir now holds the raw user value (e.g. 'TD').
• getData(): normalization is applied inline when constructing the dagre layout node — dagre still receives the canonical 'TB' it requires:

dir: subGraph.dir === 'TD' ? 'TB' : subGraph.dir, // normalize TD→TB for dagre

TD is a user-facing alias; TB is dagre's internal canonical form. These two concerns now live at the correct layers.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added necessary unit/e2e tests.
• 📓 have added documentation. Make sure MERMAID_RELEASE_VERSION is used for all new features.
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