Refactor Linux and QNX templates into thin toolchain assembly layers #87

What

Refactor the Linux and QNX toolchain templates so they primarily act as thin assembly layers for:

• action configs
• tool selection
• compiler and ABI identity
• platform/runtime attributes
• shared feature bundle composition
• platform-specific feature bundle composition
• consumer-injected feature composition

Acceptance Criteria (DoD)

• Linux and QNX templates are reduced to thin assembly-oriented implementations.
• Shared feature logic has been removed from direct template ownership where appropriate.
• Platform templates still clearly own platform/runtime/toolchain identity concerns.
• The resulting structure is easier to read and reason about than the current duplicated implementation.

How

• Update both templates to consume shared and platform-specific feature bundles.
• Keep action configs, tool paths, compiler identity, and platform metadata in the platform template layer.
• Avoid changing consumer-facing extension points unless required by the new composition structure.
• Refactor incrementally to preserve reviewability and reduce behavioral risk.