fix(linter): tighten Zig doc comment detection

[xen0n]

Summary

• only treat the immediately preceding Zig comment as the doc comment for a declaration
• avoid inheriting an earlier /// block across an intervening regular // comment
• add focused Zig tests covering documented and non-documented functions and struct declarations

Root cause

• the previous Zig detector walked backward across multiple comment siblings, so a declaration could incorrectly inherit an earlier /// doc block even when a regular // comment intervened

Testing

• cargo test -q detect_zig_doc_comment_ -- --nocapture
• cargo test -q tree_path::lang_zig::tests:: -- --nocapture