Add Mumpix Runtime Extensions v0.4 specification

[hexadecimal-REBOOT]

Motivation

• Capture and publish a formal, in-repo technical specification for the Mumpix runtime extensions (v0.4 pre-build final) so implementation teams have a single authoritative reference.
• Surface the spec from the project landing page so contributors can discover architecture, invariants, interfaces, and the phased build checklist.

Description

• Add a new comprehensive specification file at docs/mumpix-runtime-extensions-v0.4.md containing architecture, invariants, core types, module layout, interfaces, routing tiers, dream engine rules, idempotency/key normalization, metrics, and a multi-phase build checklist.
• Update README.md to include an “Additional specification” pointer linking to the new docs file for discoverability.
• No runtime source code or tests were modified in this change; this PR only adds documentation and a README link.

Testing

• Verified the new documentation file is present and measured its length with wc -l docs/mumpix-runtime-extensions-v0.4.md README.md, which completed successfully.
• Confirmed the README update by inspecting the end of README.md with nl/sed to ensure the docs link appears, and the check completed successfully.
• No unit or integration tests were added or executed as part of this documentation-only change.

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Codex Task