tool/pputil: LoadIncludes enhancement

[xushiwei]

LoadIncludes is used to list all #include directives within a C/C++ header file. The current implementation is rudimentary, simply searching for the #include string and processing it. It needs to be updated to a more robust implementation that:

• Allows spaces or tabs between # and include;
• Ignores #include occurrences found within comments or string literals.
  Please implement it as efficiently as possible.

[xushiwei]

/code

[fennoai]

Done

Enhanced LoadIncludes in tool/pputil and opened #885 against main.

What changed — rewrote LoadIncludes as a single-pass scanner (scanIncludes):

• Allows spaces/tabs between # and include (e.g. # include, #\tinclude).
• Requires # to be the first non-blank character on the line (C/C++ preprocessor rule), so a mid-expression #include is no longer matched.
• Ignores #include inside // line comments and /* ... */ block comments (including multi-line).
• Ignores #include inside "..." string and '...' char literals, honoring \ escapes.
• Drops the previously mutated package-level include slice; parsing no longer touches shared state.

The scan is a single O(n) pass over the file bytes, allocating only the yielded filename strings.

Verification

• Added tool/pputil/pputil_test.go (spaced directives, leading whitespace, line/block comments, string/char literals, non-line-start #, missing separators, #includes/#include_next substrings, unterminated names, EOF without newline, early iterator stop, and LoadIncludes I/O success/error).
• go test ./tool/pputil/ — all pass. go vet, gofmt -l, and go build — clean.