Backlog and roadmap cleanup

[sqt]

No description provided.

[greptile-apps]

Greptile Summary

This PR reorganizes the project backlog, roadmap, release history, and design documentation without changing executable behavior.

• Adds a changelog and dedicated design-decision documents.
• Replaces the oversized roadmap with a cleaner current-work plan.
• Updates implementation and backend documentation to reflect shipped capabilities.
• Refreshes source comments and documentation links after the reorganization.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
docs/roadmap.md	Replaces the accumulated roadmap with a substantially shorter, structured description of remaining work and known gaps.
CHANGELOG.md	Adds a release-oriented history of notable features and fixes.
docs/decisions.md	Records stable allocator, backend-policy, and C-ABI identity decisions previously embedded elsewhere.
docs/features.md	Refreshes backend capability tables to match the current implementation.
docs/implementation_status.md	Updates implementation notes, known limitations, and cross-references to the reorganized documentation.
docs/design/binding-c-abi-identity.md	Extracts the C-ABI cross-view identity design and its remaining limitations into a focused document.
src/backend/java.zig	Updates comments describing implemented Java bitset and JNI marshaling behavior; executable logic is unchanged.
src/backend/zig.zig	Redirects source comments to the new design documents without changing generated output.
packages/zidl-cdr/include/zidl_cdr.h	Corrects a documentation reference in the allocator API comment without changing the public API.

_{Reviews (2): Last reviewed commit: "resolving review comments" | Re-trigger Greptile}