[codex] Split build-script feature resolution

[dzbarsky]

Cargo resolver v2 keeps normal-dependency features separate from build-dependency features, but rules_rs previously resolved both dependency kinds into one external rust_crate target. This change resolves target dependencies without build dependencies, seeds a second resolution over SUPPORTED_EXEC_TRIPLES, and generates either one target when both resolutions match or _target and _exec targets selected by @rules_rust//cargo/settings:use_exec_features_enabled when they differ. It pins rules_rust commit 7837073014b566291368027aa116cd03fe108049, which composes the build-script exec transition with the marker transition through config.exec().and_then(...).

The build_exec_feature_split fixture requires target_feature from a normal dependency and exec_feature from the same crate as a build dependency. The shared crate fails compilation if both features are enabled, so the fixture verifies that the target library and build script receive different feature sets.

Validation used source-built Bazel commit 0878dd724149b73a731ab83859a55a65e9b438ed: @build_exec_feature_split//:_workspace_deps passed with 1,720 actions, //rs/private:all passed 28/28 tests, cargo check --locked passed for the fixture, and Buildifier plus git diff --check passed.

Current scope: proc macros are unchanged, and seed_exec_build_dependencies unions features from all active target triples before resolving each exec triple. Exact target-specific build-script features require resolution keyed by both target triple and exec triple.