Replace ptr = ptr.offset(1) increment patterns with in-place pointer arithmetic

[Copilot]

This change removes inefficient pointer-increment assignments that rebuilt pointers via offset(1) and standardizes them to in-place increments (+= 1) in non-test Rust code. Scope is limited to production/runtime and rule output sources; tests/ remains untouched.

• libcc2rs runtime updates

  • Updated pointer progression in:
    • libcc2rs/src/rc.rs (StringIterator)
    • libcc2rs/src/inc.rs (unsafe prefix/postfix increment trait impls for raw pointers)
    • libcc2rs/src/io.rs (fread / fread_unsafe byte-copy loops)

• Rule output source updates

  • Updated generated-rule Rust sources to use in-place pointer increments:
    • rules/stdio/tgt_refcount.rs
    • rules/stdio/tgt_unsafe.rs
    • rules/algorithm/tgt_unsafe.rs

• Pattern normalization

  • Canonical replacement applied where variable self-assignment matched pointer increment via offset(1):

// before
ptr = ptr.offset(1);

// after
ptr += 1;