Docs: DO-0 add Dara JS build redesign proposal

[krzysztof-causalens]

Motivation and Context

Dara's current JS build flow relies on a generated dist/ workspace, dara.config.json, machine-dependent Node setup, and a separate UMD / auto-JS path for non-custom-JS apps.

This PR does not change runtime behavior yet. It adds an internal proposal document so the team can review a concrete redesign direction in-repo rather than keeping the design only in chat notes.

The current proposal covers:

• moving to one Dara-managed cached Node + pnpm toolchain for all apps
• removing the UMD / auto-JS path so custom-JS and no-custom-JS apps use the same pipeline
• using a platform-independent dara.lock to avoid cross-platform lock churn
• making package.json, pnpm-lock.yaml, and dara.lock the checked-in reproducibility surface for every app
• keeping .dara/, dist/, and node_modules/ as generated or installed state that should not be checked in
• keeping non-ejected apps simple by having users update the Dara-owned JS surface through dara lock
• making dara eject the point where the root JS workspace becomes user-owned, with a warning and a narrow merge of Dara-required dependencies
• exposing Dara's Vite integration through a small @darajs/vite-plugin boundary so ejected configs stay stable and simple
• validating only the Dara-managed dependency/toolchain surface during local dev, with clear remediation instead of making Dara the owner of unrelated user dependencies
• migrating away from dara.config.json via compatibility, warn, and enforce phases, with deterministic handling of legacy package-manager settings

Implementation Description

Added a new top-level internal docs area and a draft design proposal at docs-internal/proposals/dara-js-build-redesign.md.

The proposal is intentionally opinionated enough to be actionable, but still leaves a few explicit open questions around the compatibility window for dara.config.json and where ejected source/config files should live by default.

This PR only updates internal documentation. The earlier draft changelog note was removed because this proposal is not a user-facing package change.

Tradeoffs:

• The draft proposes shipping only a limited set of managed Node targets initially.
• The draft now favors one always-managed cached toolchain over reusing user-installed Node/pnpm, prioritizing reproducibility and ease of support over flexibility in toolchain sourcing.
• The default path exposes root package.json and pnpm-lock.yaml as checked-in files for reproducibility, while dara lock remains the normal way for less experienced users to refresh them.
• Ejection gives experienced users a normal JS workspace and Vite config, with Dara's framework integration kept behind a plugin boundary.
• The PR title still uses a temporary DO-0 placeholder because no real ticket identifier was provided, but the repo's PR lint requires a ticket-shaped prefix.

Any new dependencies Introduced

None.

How Has This Been Tested?

• Reviewed the rendered markdown content locally.
• Ran git diff --check.

No runtime tests were run because this PR only updates an internal proposal document.

PR Checklist:

• I have implemented all requirements? (see JIRA, project documentation).
• I am not affecting someone else's work, If I am, they are included as a reviewer.
• I have added relevant tests (unit, integration or regression).
• I have added comments to all the bits that are hard to follow.
• I have added/updated Documentation.
• I have updated the appropriate changelog with a line for my changes.

Screenshots (if appropriate):

N/A