docs: prepare Version 2.1.0 extension foundation

[sanskarIN]

Summary

Prepares the next release line without changing the active Version 2.0.12 package/build identity.

• plans Version 2.1.0 as a platform-integration/browser-extension foundation release;
• documents Manifest V3 Chromium and Firefox WebExtensions architecture boundaries;
• establishes a least-privilege, local-only extension policy with zero default host/browser permissions;
• adds permission-free Chromium and Firefox manifest templates, clearly marked as preparation-only;
• adds a machine-readable next-version contract and atomic migration checklist;
• preserves Android, iOS, Web/PWA, Windows, macOS, and Linux as first-class maintained targets;
• records preparation continuity separately so current Version 2.0.12 evidence is not rewritten.

Important activation boundary

This PR does not bump pubspec.yaml. Current marketing/package identity remains 2.0.12 / 2.0.12+2012, and manual qualification remains 0/13. The planned 2.1.0 package is activated only after the current Phase 34 maintenance base is clean/integrated and the version-coupled surfaces can be migrated atomically.

The extension files are templates, not a supported or publishable extension package. No Chrome Web Store, Firefox Add-ons, real-browser load, extension CSP, accessibility, update/restart, or store-review evidence is claimed.

Dependency

This is a stacked preparation PR targeting maintenance/phase-34-audit-hardening. It must be synchronized onto the final Phase 34 head before integration, then retargeted to main after Phase 34 lands.