Make bar generation bound-safe

[jserv]

This replaces strncat-based appends in gen_hist_bar with bounded memcpy while tracking remaining capacity and always NUL-terminating.

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Summary by cubic

Hardened histogram bar rendering by replacing strncat with bounded memcpy, tracking remaining capacity, and always NUL-terminating. Prevents buffer overruns when writing UTF-8 block glyphs and safely handles zero-length buffers.

• Bug Fixes
  • Append full and partial block glyphs only when there’s room for the glyph plus NUL; stop otherwise.
  • Early return if buffer length is 0; ensure final NUL termination in all paths.