docs: add v3->develop merge review and create tag v3.0.0

[Sendipad]

Motivation

• Capture a concise reviewer-facing summary of the v3 -> develop merge and record release guidance for v3.0.0.
• Provide a short risk assessment and suggested post-merge smoke checks for high-impact areas (setup flow, party merge/health logic, migration backfill).
• Create an annotated release tag to mark the merge point for downstream releases and CI workflows.

Description

• Added review notes at docs/reviews/v3-to-develop-review.md summarizing scope, functional highlights, risks, and recommended smoke tests.
• Created an annotated Git tag v3.0.0 pointing to merge commit 2e374bd with message Release v3.0.0.
• Committed the review file with commit message docs: add review notes for v3 merge.
• The merge commit metadata referenced in the review shows 74 files changed, 10,549 insertions, 2,910 deletions, covering setup, data-quality, transaction-health, party issues, i18n, CI updates, and a v3 blueprint document.

Testing

• Ran git show --stat --oneline --no-color 2e374bd to inspect the merge commit statistics, which completed successfully.
• Created the tag with git tag -a v3.0.0 2e374bd -m "Release v3.0.0" and verified it with git show --no-patch --decorate v3.0.0, which succeeded.
• Added and committed the review file using git add and git commit -m "docs: add review notes for v3 merge", which succeeded and created the new commit on the current branch.
• Verified repository status with git status --short --branch, which showed the working tree on branch work as expected.

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Codex Task