[dajiaohuang]

Problem

The predefined-macro check in each elsif loop uses the identifier from the opening ifdef/ifndef, rather than the identifier on the current elsif. This changes which valid source reaches the parser:

`ifdef UNDEFINED
module first; endmodule
`elsif __FILE__
module selected; endmodule
`else
module fallback; endmodule
`endif

parse_sv_str currently returns a syntax tree containing fallback, although __FILE__ is predefined and selected should be chosen. Conversely, after ifndef __FILE__, an elsif UNDEFINED is incorrectly treated as true. Both __FILE__ and __LINE__ are affected.

Change

• Check elsifid for predefined status in both conditional-directive paths.
• Add public parse_sv_str regression tests asserting the resulting module identifiers across both builtins and both opening directives.
• Correct the existing FILE/LINE fixture comments and expected output: their elsif UNDEFINED branch must not be emitted. Source line counts are unchanged.

Validation

• Both new public-API tests fail against the unchanged production base and pass with this fix.
• cargo test passes on Ubuntu 24.04 / stable Rust 1.98.1: 127 passed, 1 pre-existing ignored.
• Separate public-API file/string/preprocessed parsing, isolated include/macro expansion, AST traversal and source-origin checks pass; both CLI examples exercised.
• Initial native Windows workspace tests encountered existing fixture line-ending/OS-error-message differences; full Windows acceptance is not claimed. Linux tests used canonical Git LF fixtures.