docs: add themed sprint roadmap for research backlog

[ChuxiJ]

Summary

• sync the completed research pack from claude/research-daw-synths-KU2At into .llm/research/
• add a master roadmap that clusters backlog issues #942-#975 into 8 themed sprints
• add one executable sprint plan per theme with user stories, root causes, issue mapping, PR slices, verification, and file targets

Why

The research work is complete, but the backlog is still too wide to execute safely as one stream. This PR turns the research into an implementation program that can be shipped theme-by-theme with reviewable PR slices.

Sprint Sequence

1. Sprint 01 — Synth Foundation
2. Sprint 06 — Onboarding, Library, and Accessibility (parallel lane)
3. Sprint 02 — Sampling, Drum Rack, and Render Pipeline
4. Sprint 03 — Mixer, Device Graph, and Routing
5. Sprint 04 — MIDI, Rhythm, and Automation Editors
6. Sprint 05 — Arrangement and Recording Workflows
7. Sprint 07 — Collaboration, Cloud, and Project Lifecycle
8. Sprint 08 — Advanced Sound Design Lab

Notes

• planning and research only; no runtime source changes in this PR
• each sprint doc includes proposed implementation PR slices so follow-up PRs can start immediately
• branch follows the repo PR-driven workflow and keeps main untouched

Validation

• verified roadmap and sprint documents are present under docs/plans/
• verified research files are present under .llm/research/
• no build/test run because this PR is documentation-only

[ChuxiJ]

Triage: Informational only — Sprint roadmap docs PR. Low priority, can be merged whenever convenient with no conflict risk.

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