Move compiler builds and prerequisite checks into tools.ci.build

[LunaStev]

Motivation

Compiler and LLVM setup procedures are repeated in .github/workflows/rust.yml and .github/workflows/release.yml; missing tools such as ripgrep already caused a CI failure in #817.

Scope

Depends on #824. Implement tools/ci/build.py around the shared Target model. Validate host/tool architectures, Rust/LLVM versions, required linkers, sysroots and tools, then build with the selected backend features and --locked --jobs 2.

Use the existing pinned Windows provisioning and LoongArch/RISC-V helpers where appropriate. Python owns prerequisite decisions and invokes platform installers/helpers; YAML may retain checkout, runtime setup, caches and runner selection. Keep verification separable from explicit provisioning so a local command does not unexpectedly install system packages. Difficulty: hard.

Completion criteria

• Local and hosted builds use the same target-specific build plan and explicit LLVM/std inputs.
• Missing/mismatched prerequisites fail before dependent compilation with an actionable diagnostic.
• Feature-isolated wasm, LoongArch, RISC-V, x86 and AArch64 builds remain covered, including Windows native host checks.
• Mocked command-plan tests and representative native CI runs verify arguments, paths, environment propagation and failed provisioning.