Add Canton Network Indexer proposal

Document the proposal for building a production-grade indexing infrastructure for the Canton Network, detailing objectives, technical approach, milestones, funding request, and team qualifications.

Development Fund Proposal Submission

Proposal file: Link to the proposal added in this PR (e.g., /proposals/your-proposal-name.md)

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Summary

Briefly describe the proposal and the value it delivers to the Canton ecosystem (2–3 sentences).

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Checklist

• [ ] Proposal file added under /proposals/
• [ ] Milestones and funding amounts defined
• [ ] Acceptance criteria included
• [ ] Alignment with Canton priorities described

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Notes for Reviewers

(Add anything the Tech & Ops Committee should pay attention to.)