Share one path component iterator across syscall/

[henrybear327]

path.c and sidecar.c each carry a private copy of the same '/'-separated component walk, differing only in name and return type.

Promote path.c's path_next_component to external linkage and switch sidecar.c to it so the syscall layer keeps a single walker and future path predicates do not grow another copy.

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Summary by cubic

Deduplicated path component iteration in the syscall layer by exporting path_next_component and switching sidecar.c to use it. This keeps one '/'-component walker and aligns behavior across callers with no functional changes.

• Refactors
  • Exposed path_next_component(const char **pathp, const char **comp, size_t *len) in path.c and declared it in path.h with docs.
  • Removed sidecar_next_component and updated call sites in sidecar_translate_lookup_at to use the shared helper.
  • Preserved semantics: skips repeated slashes and returns false at end of path.

^{Written for commit 3451457. Summary will update on new commits.}

[jserv]

Thank @henrybear327 for contributing!