research: bring the v5 experience research to main #403

Why

Move codex/v5-experience-research to docs/proposals/v5/ and re-read it against project-only management.

In scope

Intake: problem, outcome, size range, dependencies.

Out of scope

Implementation.

Acceptance criteria

• Problem, outcome, size range and dependencies recorded

Dependencies

Blocked by: None

Blocks: None

Plan

• Release: 5.0.0
• Size: size:M · Workstream: ws:v5-research
• Epic: research: v5 planning intake (epic) #306
• Card: V5-02 in the card inventory
• Sources: docs/proposals/v5-planning-sources.md; v5-impact-of-v4.md

Verification

Each acceptance criterion is checked on the pull request.