docs(sdd): localfirst-infra-v1 PRD + SDD artifacts

[kattsushi]

Goal

Land the PRD + SDD phase artifacts for localfirst-infra-v1 so implementation can proceed in chained PRs with clear scope, requirements, design, and task breakdown.

What’s in this PR

• PRD: docs/prd/localfirst-infra-v1.md
• SDD artifacts:
  • openspec/changes/localfirst-infra-v1/proposal.md
  • openspec/changes/localfirst-infra-v1/spec.md
  • openspec/changes/localfirst-infra-v1/design.md
  • openspec/changes/localfirst-infra-v1/tasks.md

Key decisions captured

• Local-first/client-first, Effect-first foundation slice; Notes is a placeholder bounded context.
• Chrome/Chromium first.
• SQLite OPFS in Worker via @effect/sql-sqlite-wasm as reversible spike.
• effect-qb beta as reversible spike; don’t couple upper layers.
• LocalDb is infra-only behind repositories.
• Presentation state uses Effect Atom + Solid integration.
• Migration authoring DX (Nx generator vs effect-qb CLI vs hybrid) is pending spike results.

Review path

1. Read PRD first.
2. Validate spec scenarios are verifiable.
3. Confirm design matches repo conventions.
4. Confirm tasks + 5-PR chain forecast look reasonable.

Non-goals (this PR)

• No implementation code.

Next PRs (planned)

• PR 2: Spikes + decisions (Worker/OPFS/WASM, effect-qb viability, migration DX decision)
• PR 3: localfirst-db core (contract + in-memory + migrations)
• PR 4: localfirst-db browser adapter (Worker + OPFS + BroadcastChannel)
• PR 5: notes bounded context core
• PR 6: notes presentation + app wiring + boundary enforcement + verification