feat(scaffold): honor per-target core-dependency overrides in the scripting bindings

[tobocop2]

Fixes #163.

What this changes

alef generates a Rust crate for each language binding and writes its Cargo.toml. You can tell alef to build the core crate with different features on a specific platform, via target_dep_overrides in alef.toml — but until now only the FFI, JNI, Dart, and Swift bindings honored it. The Python, Node, Ruby, PHP, and Elixir bindings ignored it and always wrote a single, platform-independent core dependency.

This makes those five bindings honor it too.

Before — a Python binding's Cargo.toml, override silently ignored:

[dependencies]
core-lib = { version = "1.0", path = "…", features = ["full"] }

After — same binding, with an override configured for Intel macOS:

[target.'cfg(not(all(target_os = "macos", target_arch = "x86_64")))'.dependencies]
core-lib = { version = "1.0", path = "…", features = ["full"] }

[target.'cfg(all(target_os = "macos", target_arch = "x86_64"))'.dependencies]
core-lib = { version = "1.0", path = "…", features = ["macos-intel-target"] }

How

The FFI backend already does exactly this, so the change is a straight port, not new design:

• moved the "split the core dependency into per-target sections" logic into a shared helper (render_core_dep_with_overrides);
• added the target_dep_overrides option to the Python/Node/Ruby/PHP/Elixir configs (reusing the FFI backend's FfiTargetDepOverride, as JNI already does), which regenerates the JSON schema;
• wired each of the five scaffolds to use the helper.

The override sets the features for the matched platform (same shape as FFI/JNI). The default_features = false variant that Dart/Swift also support is not included here — it can be a follow-up if needed.

Verification

cargo fmt --check and cargo check are clean. New tests cover the helper directly and assert that all five bindings emit the per-platform sections. The full library test run adds no failures: origin/main is 4181 passed / 14 failed, this branch is 4184 passed / 14 failed — the same 14 pre-existing, environment-sensitive test_scaffold_* failures (they read the real workspace Cargo.toml), plus the 3 new passing tests.

Follow-up commit ebcbaaa extends the new config field to the struct literals in the tests/ integration suites (backends_pyo3_gen_bindings/*, backends_napi_gen_bindings_test.rs), so --all-targets compiles cleanly — the Validate check is green as of that commit. The red Test / poly-validate checks reproduce identically on main (the 14 environment-sensitive failures above, and repo-wide poly fmt drift in files this PR doesn't touch).