Update platform boundary docs for Base v1.6 Ubuntu setup

[codeforester]

Context

base-demo currently documents Ubuntu/Linux as a read-only CI path, which is correct for the pinned Base v1.5.0 contract. Base v1.6.0 is expected to add apt-backed Ubuntu/Debian setup and dev-profile behavior. Once base-demo CI proves that path, the docs and contract registry should reflect the new boundary precisely.

Scope

• Depends on Pin base-demo CI to Base v1.6.0 #159 and Add Ubuntu apt-backed Base setup validation for v1.6 #160.
• Update README.md Platform Requirements and Quick Start notes for the tested Base 1.6 Ubuntu boundary.
• Update CONTRIBUTING.md validation guidance.
• Update docs/contracts.md if the platform-boundary or ubuntu-ci contracts change.
• Update tests/validate.sh guards so docs cannot drift from the new tested contract.
• Keep macOS as the full interactive demo platform unless Ubuntu setup, activation, build/test, and non-interactive demo are all proven in CI.

Validation

• ./tests/validate.sh
• git diff --check
• CI validate and validate-ubuntu pass.

Dependencies

• Pin base-demo CI to Base v1.6.0 #159 must land first.
• Add Ubuntu apt-backed Base setup validation for v1.6 #160 must establish exactly which Ubuntu setup path is tested.

Expected wording direction

The docs should distinguish:

• macOS full interactive demo support;
• Ubuntu/Debian Base setup support as proven by Base v1.6.0;
• base-demo project health/read-only validation on Ubuntu;
• any remaining Linux exclusions for Brewfile, Homebrew artifacts, IDE setup, activation shells, services, or Docker if those are not proven.