docs: align Base ecosystem platform, license, and release-policy contracts

[codeforester]

Goal

Align base-demo documentation with the canonical Base ecosystem platform, license, and release policy.

Background

The parent cross-repository policy issue is basefoundry/base#2116. base-demo remains an independently released MIT reference consumer. Its full interactive path and its Ubuntu read-only validation path must be distinct from component-level support and the coordinated Base release matrix.

Scope

• Link the README, contributing guide, contracts, and release guide to the canonical Base policy.
• Clarify that WSL2 is limited read-only/development guidance and is not in the coordinated Base release matrix.
• Keep native Windows out of the base-demo contract.
• Clarify the independent MIT license and immutable release behavior.
• Update the affected documentation assertion.

Acceptance Criteria

• Current docs clearly distinguish macOS full-demo, Ubuntu read-only, WSL2 guidance, and native Windows out of scope.
• The independent MIT license and release-asset immutability are explicit.
• The documentation validator enforces the revised WSL2 wording.
• Full validation and diff checks pass.

Validation

• GRADLE_USER_HOME=/private/tmp/base-demo-gradle-2116 PATH=:$PATH ./tests/validate.sh
• git diff --check

Non-Goals

• Do not bump the pinned Base or base-bash-libs consumer versions; that belongs to the downstream bump train.
• Do not change the demo runtime or service contracts.

Project Fields

• Status: Backlog
• Priority: P2
• Size: S
• Area: Docs
• Initiative: Adoption Polish

Ownership

• Assignee: @codeforester

Parent: basefoundry/base#2116

[codeforester]

Closed after companion PR #284 merged. The ecosystem platform, license, and immutable release-policy documentation is aligned with Base #2116; the full demo validator passed.