build: move linking onto root modules

[forketyfork]

Issue

Prepare Architect for Zig 0.16 by moving build linkage and include configuration off std.Build.Step.Compile, where these APIs are removed.

Solution

• Set link_libc = true on the exe_mod and mcp_mod root modules.
• Move SDL3, SDL3_ttf, macOS framework, include-path, library-path, and SDK framework configuration onto exe_mod.
• Read build environment values through b.graph.env_map.
• Remove the redundant MCP test-step libc link.

Context

This is exactly Step 2 of the Zig 0.16 migration plan and remains compatible with the repository's Zig 0.15.2 toolchain. Compile-level headerpad_max_install_names settings remain on the executable steps.

Test plan

• zig build passes under the installed Zig 0.15.2 toolchain.
• Unpiped zig build test passes.
• just lint passes, including test-registry and Zwanzig checks.
• The Step 2 grep gates report no Compile-level link/include calls and no std.posix.getenv calls in build.zig.
• The prescribed nix develop commands could not run because Nix is unavailable in this Linux sandbox; equivalent direct commands were run.
• macOS-only otool -L SDL/framework checks were not run because this is Linux. The app launch was attempted but SDL could not initialize because the sandbox has no available video device; manual window verification still needs a macOS environment.