Align native sequence iterator state and lifecycle - #9701

Summary

Align concrete sequence iterator state and lifecycle with CPython 3.15 in 437 changed production Rust lines across nine files.

• Share signed cursor, owner retention, length hints, state validation and reduction snapshots through a native iterator helper. Concrete reads remain atomic without holding locks across Python allocation or owner release.
• Preserve the distinct list, reverse-list, immutable sequence and bytearray negative-state rules, including reversible list states and irreversible bytearray exhaustion.
• Validate strict integer state and native ssize overflow even after exhaustion. Share the borrowed conversion helper with the generic iterator companion in Align builtin iterator controls and callback lifetimes #9699, rather than duplicating conversion logic.
• Correct empty reverse-list initialization, clamping and live length hints. Keep list/tuple fast paths and normal iteration on the same implementation.
• Move bytes, bytearray and string result allocation outside iterator/storage locks, keep rejected string states from invalidating cached offsets, and register the retained owners for cyclic traversal.

The separate memoryview lifecycle changes extend #9692, which already owns memory iterator constructor validation. This PR does not duplicate that work. Existing generic sequence, dictionary/set, range and other PositionIterInternal consumers remain unchanged. No Python, Lib, tests, expected-failure markers or workflows changed.

Reference and validation

Source comparison uses CPython commit d220409a13b77de7a6c30abe16ea90db01528edc: list, tuple, string, bytes and bytearray.

• 60,141 bounded CPython/source-model checks across 1,500 randomized state traces, plus independent strict-state, finalizer, traversal and allocation-order reference probes. These establish reference behavior and review the source translation; they are not RustPython runtime tests.
• Independent review of the final nine source files and the two separate companions. Actual source compositions with Align bytearray mutation and export controls #9694, Align list, tuple and slice controls #9696, Align text and bytes argument and buffer controls #9698 and Align builtin iterator controls and callback lifetimes #9699 preserve the existing changes and a single shared strict-size helper.
• Focused host-env/VM/stdlib threading cargo check and normal all-targets Clippy passed. Four diagnostics match the unchanged main baseline exactly: one POSIX needless-collect and three ElementTree lints. No warnings were suppressed; strict -D warnings remains blocked by that baseline.
• Formatting, diff checks and normal commit hooks passed.

Native RustPython execution was not performed. Full workspace/C-API suites and cross-platform runtime tests remain unrun. Hosted CI is required, and no runtime expected failure is claimed resolved. Existing cross-thread scheduling and allocation-timing differences outside these iterator controls remain outside scope.

AI-assisted implementation and independent review using OpenAI Codex; model version is not exposed.