refactor: Rewrite assert statements as exceptions

[rapsealk]

Summary

Mechanical sweep of assert statements in library code, rewriting them as if not cond: raise AssertionError[(msg)] so the checks still fire under python -O (which strips assert). Picks up the task described in #1408.

• Scope: 37 sites across bitsandbytes/ (36) + check_bnb_install.py (1).
• Out of scope: tests/ (pytest relies on assert for introspection) and examples/ (didactic code).
• Approach: mechanical rewrite only — no semantic exception upgrades (ValueError/TypeError/etc.), since the issue is labeled Low Risk and per-site judgment would enlarge review surface. Happy to follow up with semantic upgrades in a separate PR if maintainers prefer.

Audits run before the rewrite

• Tuple-trap audit (assert (cond, msg) — always-truthy bug): none found.
• Side-effect audit (assert fn(...) patterns that would be skipped under -O): all hits were pure queries (isinstance, len, .numel(), set().issubset, P.is_parametrized) — no state-mutating calls, so no behavior regression from "now runs under -O".

Test plan

• grep -rnE "^\s*assert\b" bitsandbytes/ check_bnb_install.py returns empty (0 asserts remain in scope)
• python3 -m py_compile on all 9 touched files
• ruff check bitsandbytes/ check_bnb_install.py — all checks passed
• ruff format --check bitsandbytes/ check_bnb_install.py — all files already formatted
• pre-commit run --files ... with the repo's pinned ruff 0.14.3 — all hooks pass (ruff, ruff-format, EOF, trailing whitespace, line endings, typos)
• python -c "import bitsandbytes" — not run locally (no torch on my machine). CI tests-pr.yml covers this.
• pytest --collect-only tests/ — same; relying on CI.

Closes #1408.

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Assisted by Claude Code.