Build: support Zig 0.17.0-dev.1811 ArrayList API

Parent dependency: zig-utils/zig-js#626

Problem

Zig 0.17.0-dev.1811 removed ArrayList.getLast, so src/backtrack.zig no longer compiles at the greedy repetition result even though the list is provably non-empty after its mandatory initial append.

GitHub Actions evidence: zig-utils/zig-js/actions/runs/32258993747 reports src/backtrack.zig:991:31: error: no field or member function named 'getLast'.

Scope

• Replace the removed convenience API with an explicit final-item access whose non-empty invariant is visible at the call site.
• Preserve greedy match semantics, capture rollback, allocation-failure behavior, and compatibility with the repository's supported Zig 0.17 development floor.
• Audit the owned source for any other getLast uses.

Acceptance

• Focused and full zig-regex tests pass on the local supported compiler.
• Analyze/compile succeeds with Zig 0.17.0-dev.1811.
• The dependent zig-js build no longer reports the sibling error.

Implemented and pushed to main in 87e9ca2.

Validation:

• zig build test on Zig 0.17.0-dev.1441+d5181a9c9
• zig build test on the exact CI compiler, Zig 0.17.0-dev.1811+6716bf52e

The greedy path retains its non-empty invariant and longest-match result while avoiding the removed ArrayList.getLast API.