refactor: Split engine builder modules (#13009)
## Why
The engine builder had grown large enough to make focused reviews and
AI-assisted context difficult. Splitting it into cohesive modules keeps
the core builder entry point small without changing behavior.
## What
Extracts task-definition resolution, task inheritance resolution, and
builder unit tests into sibling modules under
`crates/turborepo-engine/src/builder/`. Relocates the affected insta
snapshots to match the new test module path.
## How
This is intended as a mechanical refactor. Verified with `cargo fmt -p
turborepo-engine`, `cargo test -p turborepo-engine`, `git diff --check`,
and the repository pre-push hook, including `turbo run format
check:toml`, `cargo fmt --check`, `cargo lint`, and `cargo check
--workspace`.
