docs(spec): add V2 New Spec Impact review; tidy V2 Auth nav menus #1605

Primary — V2 New Spec Impact

Add specification/v2_new_spec_impact.md: a review of the MCP 2026-07-28 SEPs (transport/HTTP, authorization, extensibility/UI, governance) and their concrete impact on Inspector v2 (v2/main) — the connection/era model, History & Network tabs, feature tabs, the OAuth store, new error codes, and suggested sequencing. Wired into the top-level spec navigation.

Secondary — Auth nav tidy

• New specification/v2_auth.md landing page summarizing the four v2_auth_*.md specs (EMA / XAA, Hardening, Mid-session, Smoke testing) with a section submenu, mirroring the V2 UX index.
• Uniform top menus: added a V2 Auth entry (after V2 UX) and a V2 New Spec Impact entry to every spec page's top menu.
• The four v2_auth_*.md pages moved onto the shared two-tier menu (top menu + auth submenu).

Documentation only — no code changes.