docs(playbook): v1.1 — service-locator row + coupled-writes subsection

[hyperpolymath]

Summary

Two playbook additions surfaced by the idaptik-user-settings design walk-through that landed in #36:

1. Service-locator-globals row added to the ReScript pattern index. Module.get(): Option<Thing> is a common ReScript pattern; lifting it to an effect makes the dependency visible at the call site and lets tests install a mock without monkey-patching a global. Without this row, the playbook covered ref and let mutable but missed the service-locator one level above.

2. Coupled-writes subsection added to Decision Criteria. When one user action must update both an in-memory subsystem AND persist to storage (volume, theme, settings), the choice is between one function calling two effects (default — keep coupling local) and a wrapper effect (only when partial success is a bug). The rule tracks the same logic as mut vs State.

Why stacked on #36

Both gaps were identified inside #36's lesson but deferred so that PR could land focused as policy. This PR is the v1.1 follow-up. It targets docs/migration-playbook so reviewers see only the v1.1 delta; once #36 merges, GitHub will retarget this PR to main automatically.

What's in this PR

• docs/guides/migration-playbook.adoc:
  • :revnumber: 1.1, :revdate: 2026-05-02
  • New row in the ReScript source-pattern index for module-level singleton getters
  • New "Coupled writes — multiple effects in one action" subsection in Decision Criteria
  • New v1.1 row in the Revision History appendix
• docs/guides/lessons/migrations/idaptik-user-settings.adoc:
  • Retires the "deferred to v1.1" note now that v1.1 exists
  • Cross-references the two new playbook sections

No removals; existing v1.0 guidance unchanged.

Test plan

• asciidoctor --backend=html5 --safe-mode=safe renders both files cleanly.
• New section markers (service-locator, coupled writes, "both succeed or neither") present in rendered HTML.
• Lesson cross-references resolve to the new playbook sections.
• Optional: review whether the wrapper-effect example in Coupled Writes wants a code snippet.

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