feat: per-crate rustc flag configuration trimming

[rejuvenile]

Summary

Add a skip_per_crate_rustc_flags attribute and transition to clear the experimental_per_crate_rustc_flag setting for targets that would never match any per-crate flag filter, improving cache hit rates for third-party crates.

Problem

When experimental_per_crate_rustc_flag is set (e.g., to pass -Copt-level=2 to first-party crates matching //.*@), all transitive third-party crates are placed into a different Bazel configuration even though the flag filter never matches them. In a typical project with hundreds of third-party crates, this means:

• Changing any per-crate flag causes a full rebuild of all third-party crates
• Remote cache hit rates drop because the configuration hash changes
• Build times increase proportionally to the number of third-party crates

Solution

Add a configuration trimming mechanism:

1. New attribute: skip_per_crate_rustc_flags (bool, default False) on all Rust rule types (rust_library, rust_binary, rust_test, rust_proc_macro, rust_static_library, rust_shared_library)

2. New transition: per_crate_flag_trim_transition in rust/private/per_crate_flag_trim.bzl — when skip_per_crate_rustc_flags = True, clears the experimental_per_crate_rustc_flag setting, returning the target to a canonical configuration

3. Integration: The transition is applied via cfg on rust_library and rust_proc_macro, and integrated into existing platform transitions for binary/test/static/shared library rules

4. crate_universe: The renderer sets skip_per_crate_rustc_flags: true on all generated third-party crate targets

How it works

first_party_binary (per_crate_flags = ["//.*@-Copt-level=2"])
  └── first_party_lib (flags match → -Copt-level=2 applied)
  └── serde (skip_per_crate_rustc_flags = True)
        → transition clears per_crate_flags → canonical config
        → cache key is stable regardless of flag changes

When skip_per_crate_rustc_flags = False (the default for first-party crates), the transition is a no-op and preserves the current behavior.

Changes

• rust/private/per_crate_flag_trim.bzl (new): Transition implementation
• rust/private/rust.bzl: Add attribute, wire transitions
• crate_universe/src/rendering.rs: Set attribute on generated targets
• crate_universe/src/utils/starlark.rs: Add field to CommonAttrs struct

Compatibility

• Fully backward compatible: The attribute defaults to False, so existing targets are unaffected
• No behavior change for first-party crates: Only targets explicitly opting in have their configuration trimmed
• Works with existing --experimental_per_crate_rustc_flag usage: The trim only affects the per-crate flag setting, not other configuration values