chore: prepare v1.7.0 release

[grazzolini]

Summary

Prepare the v1.7.0 release metadata and dated product changelog from the merged main branch.

Motivation

The ordered v1.7.0 product work is merged. This patch aligns package metadata and current-version fixtures before the later tagged release checkpoints. Refs #564.

Changes

• Bump all six governed version sources to 1.7.0 and update current-version health fixtures.
• Regenerate the backend and Cargo lockfiles; only their project-version entries changed, with no dependency churn.
• Promote the curated changelog to 2026-10-02, add its compare link, and clarify that packaged first-use/offline smoke and focused listening remain DrumSep release gates. Unavailable accelerator-device evidence is optional and must be reported as unverified.

Testing

• pnpm release:check-version — passed at 1.7.0.
• pnpm lint and pnpm typecheck — passed.
• pnpm test — passed; includes desktop, Tauri shell, backend, and script suites.
• (cd apps/desktop/src-tauri && cargo check) — passed.
• pnpm --filter @tuneforge/desktop test:e2e -- --run --group export --keep-artifacts — passed with isolated generated fixtures at wide and narrow viewports.
• node scripts/capture-release-media.mjs --run with temporary output — captured 21 screenshots and one silent video. Inspected every screenshot and its poster, decoded the full video, and visually checked ten frames across its duration.
• The merged-main CI rerun passed. Its earlier Export geometry failure was timing-sensitive; this metadata patch leaves the test unchanged.

Checklist

• Conventional Commit title; lint, typecheck, and full tests pass.
• No backend route or schema changes; generated API contracts are unchanged.
• Promote the existing [Unreleased] product entries into the dated 1.7.0 section.
• Update the relevant stem-separation release guidance; add no secrets, copyrighted audio, or package artifacts.
• Release Package Gate evidence: tagged builds, strict generated license inventories, packaged launch smoke, and focused listening belong to later manual release checkpoints.