Bump large numeric identifiers without integer conversion - #494

Numeric prerelease and build identifiers accepted by Version.parse can still fail when bumped: a 5000-digit identifier raises Python's integer string conversion ValueError. The comparison path already avoids that limit following #487/#488.

Increment ASCII decimal strings directly and reuse the helper for prerelease bumps, build bumps, and the old-style rc3 branch of next_version. Carry propagation works without changing the process-wide conversion limit. Existing build zero padding and constructor Unicode-digit behavior are preserved.

Regression tests cover all three paths with no carry, a carry chain, and an extra output digit; all nine fail before the fix. Full test/doctest suites pass under tox on Python 3.10 through 3.15 (498 passed, 1 skipped per interpreter). The repository-wide ruff check currently fails with existing diagnostics; comparing the unmodified source and this patch produces the same 160 diagnostics and no new diagnostics. The pr494.bugfix.rst changelog fragment is included; tox changelog check and draft build pass.