feat(cross): add --define CLI override for build-system -D defines

[jwinarske]

What

Adds a repeatable --define/-D KEY=VALUE flag to emb cross that overrides a build-system -D define for the embedder configure. The override wins over every manifest source — cross.defines, any backends entry, and anything an extends chain merged in.

How

• Parsed up front, so a malformed -D is a usage error before any download.
• Baked into the CrossTarget via withDefineOverrides, so everything downstream sees the final values: buildKey (the build-dir key differs, so caches never go stale), the backend matrix, the no-backend build, and --dry-run.
• Rides on the shared defines map, which both the CMake (cmake -S) and Meson (meson setup) paths emit as -Dkey=value — so it is generator-agnostic. cmakeArgs (CMake-only) is deliberately left untouched.

Safety

• Values reach the build tool only as discrete argv elements (Process.start with runInShell: false) — no shell interpolation.
• parseDefineOverrides splits on the first = only (a value may contain =), allows an empty value (CMake unset-style), and rejects a missing = or empty key with a FormatException.

Tests

• Unit coverage for withDefineOverrides / parseDefineOverrides (multiple overrides, verbatim spaces, empty value, generator preserved, no mutation).
• Builder-level tests proving the override reaches the CMake and Meson configure argv, and every backend, with the replaced manifest value gone.
• CLI --dry-run cases: Meson embedder, glued -DKEY=VALUE form, empty value, embedded =, and repeated-key last-wins.

Full suite green (549 tests); every new line is covered including both error branches.