Extract common explicit feature definitions into a shared module

[nradakovic]

What

Create a shared module for explicit C++ toolchain features that are genuinely common across Linux and QNX toolchains.

The goal is to remove duplicated feature logic while preserving current behavior.

Acceptance Criteria (DoD)

• A shared feature module exists for common explicit feature definitions.
• Common feature logic is no longer duplicated in both Linux and QNX templates.
• The extracted definitions preserve current behavior and feature ordering.
• The shared module is structured so additional common features can be added without duplicating logic again.

How

• Use the classification output from the feature review task as the source of truth.
• Move common feature constructors or feature bundle assembly into a shared Starlark module.
• Keep the extraction behavior-preserving and avoid changing platform-specific logic in the same step.
• Ensure the shared module has clear ownership boundaries and naming conventions.