MCP upgrade 6/6: Shared HTTP proxy

[Lauritz-Timm]

Part of the six-phase full MCP service upgrade.

Why

Multiple stdio clients should share one warm MCP daemon while retaining secure session isolation and deterministic recovery.

What

• Adds the authenticated shared HTTP MCP service and stdio bridge.
• Reuses one warm daemon across clients.
• Adds bounded sessions, recovery, shutdown, and transport coverage.

Dependency

Direct prerequisite: Phase 2 — Revision-aware core service. This phase is independently mergeable after Core.

Verification

• The clean session-recovery review approved the final minimal repair.
• HTTP integration tests passed 8/8 outside the sandbox after the sandbox-only socket-bind denial.
• Formatting, strict all-target/all-feature Clippy, and diff checks passed.
• The final isolated candidate evaluation passed 294/294 scenarios in each of two portable roots with normalized equality.