docs(changelog): release v0.27.0

Rolls [Unreleased] into [0.27.0] - 2026-10-15 and leaves a fresh empty
[Unreleased] on top. Adds the mandatory headline (exec is all or
nothing) and grouped entries for the user-facing changes since v0.26.0
that had none: the round-trip fixes from real apps, Mendix 9 writes,
the wider check -p reference resolution, filemanager, role-based
visibility, += / -= in change, the domain model SVG and the marketplace
and test-runner fixes. Moves Security to its canonical place after
Fixed. Adds a v0.27.0 release post to the docs-site blog, with the
poster and the module-role breaking change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014DfG9NpcA979LzU5R8j1Ay

Author: ako