docs(release): cover #314 stricter validation + #316 linux-arm64 in v0.8.0 notes

[aaltshuler]

The v0.8.0 release notes (docs/releases/v0.8.0.md) were written in the
version-bump commit (#313). Two user-relevant changes landed on main after
that commit, so they are currently undocumented in the notes:

• feat(engine): unify constraint validation across all write surfaces #314 — unify constraint validation across all write surfaces. The behavior
  changes are all stricter (none relaxed): enum now enforced on the merge path,
  cross-version @unique collisions now rejected, precise within-batch vs
  across-batch duplicate-@key semantics, and per-table overwrite validation.
  These are user-visible and can newly reject inputs that previously slipped
  through, so they get a dedicated "Stricter, unified constraint validation"
  section.
• ci: add linux-arm64 (aarch64) prebuilt release target #316 — linux-arm64 (aarch64) prebuilt target. Now a first-class prebuilt
  archive + Homebrew bottle; documented as a new platform.

Also adds a one-clause mention of the stricter validation to the release intro.

Docs-only; no code changes. This is the last content gap before cutting the
v0.8.0 tag from the current main tip.

🤖 Generated with Claude Code

Greptile Summary

This PR updates the v0.8.0 release notes. The main changes are:

• Added a stricter validation section for constraint checks across write paths.
• Clarified duplicate key and uniqueness behavior in the release notes.
• Added linux-arm64 as a documented prebuilt platform.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
docs/releases/v0.8.0.md	Adds release-note coverage for stricter validation and linux-arm64 prebuilts.

_{Reviews (2): Last reviewed commit: "docs(release): scope cross-version uniqu..." | Re-trigger Greptile}

Context used:

• Context used - AGENTS.md (source)
• Context used - CLAUDE.md (source)