[MISRA C] Improve not-applicable rationale for R.15.6

[PLeVasseur]

Task

Review and improve the table rationale explaining why this rule is not applicable to Rust.

Source mapping

• Standard: MISRA C
• Rule: R.15.6
• Title: TBD
• Mapping table: Table 3
• Applicability label: not-applicable-to-rust
• Source category: Required
• Existing Rust rule cell: empty
• Source file/object: src/appendices/standards-matrices/misra-c-2025-mapping.rst
• Source URL: none

Current table comment

No current comment provided.

Acceptance criteria

• Verify the not-applicable assessment against Rust semantics, std/core, unsafe Rust, and FFI where relevant.
• Replace terse or informal rationale with clear, reviewable prose.
• Add references where useful.
• Keep the mapping in Table 3 unless research shows it belongs in Table 1 or Table 2; if so, file follow-up guideline and Clippy issues instead of silently moving it.