Coordinated cross-SDK release (0.9.0) after rich-text *_attachments stack

[jeremy]

Durable tracker for the coordinated cross-SDK version bump + release that must
follow the rich-text *_attachments follow-up stack. "Scheduled separately" is
not durable tracking — this issue is.

Why a bump is needed

#433 lands a breaking change (Kotlin): 24 optional model array fields across
15 models change from List<T> to List<T>? = null. Current version is 0.8.0;
with a Kotlin-breaking change the appropriate target is 0.9.0.

Merge / release sequence

• Merge A — Hygiene: cross-ref rich-text attachments in recordable-subtypes plan #427 (hygiene: recordable-subtypes cross-ref, provenance/issue-link doc fixes). Non-breaking.
• Merge B — Fixture-completeness guard: manifest + coverage invariant #432 (fixture-completeness guard + type/nullability validation + CI job).
• Rebase D onto the resulting main — Kotlin: optional model arrays become List<T>? = null (breaking) #433. Known conflict: B and D both edit the same .PHONY line and the check: recipe line in Makefile (each appends its guard target). The rebase must retain both check-fixture-coverage and kt-check-optional-arrays. Re-run CI after the rebase.
• Merge D — Kotlin: optional model arrays become List<T>? = null (breaking) #433.
• Open the coordinated bump PR — make bump VERSION=0.9.0 (bumps all six SDKs + lockfiles + conformance runner). Do NOT bump inside A/B/D.
• Release — after bump CI is green and merged, make release VERSION=0.9.0 from clean main; monitor all release workflows (release-github, release-go, release-typescript, release-ruby, release-kotlin, release-swift, release-python).

Open follow-up debt (not blockers for this release)

• Optional scalar fields still use zero-value sentinels (SPEC Optional Fields debt) #436 — optional scalar zero-value sentinels (SPEC Optional Fields); rubric-audit.json 1B.4 is pass:false until resolved or explicitly waived.
• SearchResult.attachments: aggregate downloadable-files key unmodeled #428 — SearchResult.attachments aggregate key unmodeled (demand-gated).
• Add shared Gauge/GaugeNeedle fixtures for coverage guard (currently excluded) #429 — shared Gauge/GaugeNeedle fixtures for the coverage guard (currently excluded).

Related PRs

• Hygiene: cross-ref rich-text attachments in recordable-subtypes plan #427 (A), Fixture-completeness guard: manifest + coverage invariant #432 (B), Kotlin: optional model arrays become List<T>? = null (breaking) #433 (D — breaking)