v0.3.0 PR#1: Mock provider + EnsureAttribute step + demo workflow

[blindzero]

• Adds IdLE.Provider.Mock for deterministic demos/tests (in-memory).
• Adds step IdLE.Step.EnsureAttribute via Invoke-IdleStepEnsureAttribute.
• Fixes Invoke-IdleStepEmitEvent to be host-agnostic (EventSink/no-op fallback).
• Improves Get-IdleStepRegistry to merge host registry + built-ins consistently.
• Adds demo workflow joiner-minimal-ensureattribute and updates run-demo.ps1.
• Extends module surface tests and adds provider tests.

Closes: #7

[chatgpt-codex-connector]

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