Include labels in fast progress bar width - #331

The known-length fast bar allocates the whole terminal width to its status and bar, then adds prefix/desc and suffix. An ordinary labeled bar can overflow an 80-column terminal to 95 columns.

Reserve the measured fixed text before sizing the filler. Use the existing custom_len callback so ANSI colors and custom display widths are handled consistently.

CPython 3.13.14 / macOS arm64 validation:

• Ten label-width regressions fail before the fix; all 52 affected cases pass afterward.
• The repository's actual pip/tox route passes 1,251 tests with 16 existing skips, Pyright, and 100% configured line/branch coverage.
• Ruff, codespell, strict docs, native demo and package checks pass.
• Real PTY comparisons reproduce 95-column frames before the fix and 80 afterward. Independent PTY checks cover 80/110-column labels, ANSI text, the unchanged spinner path and short-terminal behavior.

Fixed text wider than the terminal is still not truncated. Optional custom/native formatter hooks are unchanged; other platform/runtime combinations and hosted CI remain unverified locally.