Fix Ubuntu packages and document Zig build requirements

[domenkozar]

Source builds include a terminal that requires Zig 0.16, and Linux packages have two compatibility problems: the 0.5.2 DEB requires LLVM runtime package names no longer supplied by the Ubuntu 24.04 LLVM repository, and its binary links libxml2.so.2, while Ubuntu 26.04 supplies libxml2.so.16.

Accept LLVM's current libc++1 and libc++abi1 runtime names at version 21 or newer, retaining alternatives for older versioned names. Build a separate Ubuntu 26.04 DEB in a native container against libxml2.so.16, select its matching libxml2-16 package dependency, and distinguish its filename and website download label from Ubuntu 24.04. Ubuntu 26.04 uses its native LLVM packages; Ubuntu 24.04 retains LLVM repository setup. Resolve the Zig installer through the container workspace.

Installation guidance now puts Zig and native dependencies before Cargo commands, shows installations without the terminal, and explains which DEB to choose. The docs identify the existing 0.5.2 DEB's limitations. Release drafts wait for the Ubuntu 26.04 installation check. Update the changelog.

Addresses #31 and the Ubuntu 26.04 report in this PR.

Validation:

• A clean Ubuntu 24.04 CI run reproduced the obsolete runtime names. The corrected dependency metadata passed installation, runtime-version, shared-library, and agentaps --version checks using the checksum-verified 0.5.2 binary, without Zig.
• The Ubuntu 26.04 packaging workflow builds a new binary, verifies the libxml2 SONAME, packages it, and installs the exact DEB in a fresh Ubuntu 26.04 container. It verifies checksum, shared libraries, and agentaps --version, with neither Zig nor Cargo installed. Native build and clean Ubuntu 26.04 installation passed. The candidate artifact is desktop-x86_64-unknown-linux-gnu-ubuntu26.04; its checksum and libxml2-16 control dependency were also verified locally.
• Workflow and documentation shell syntax, website JavaScript syntax, documentation links, and git diff --check passed locally. A mocked release response verified distinct Ubuntu 24.04, Ubuntu 26.04, and AppImage links and labels.
• All current standard Checks jobs passed on the updated branch: Linux/macOS/Windows desktop checks, browser/WASM checks, and the Ubuntu 24.04 installation check. The macOS release package also passed; remaining Linux/Windows release-package jobs are still running.

GUI launch and terminal rendering still need desktop smoke tests on both Ubuntu versions.

Prepared with Codex assistance.

[qrilka]

@domenkozar some more details that could be useful: Ubuntu-26 includes libc++abi1-21 and libc++21 but it lacks libxml2 (ships libxml2-16 which as I understand is not compatible), also I needed libqt6widgetst there (but got stuck because of libxml2 in the end)

[domenkozar]

@domenkozar some more details that could be useful: Ubuntu-26 includes libc++abi1-21 and libc++21 but it lacks libxml2 (ships libxml2-16 which as I understand is not compatible), also I needed libqt6widgetst there (but got stuck because of libxml2 in the end)

done :)

[qrilka]

It would be great to have a new release to check it out