[codex] Use empty checks in runtime and codegen

[anthonyprintup]

Summary

• Replace range emptiness checks in the runtime with .empty() where the type exposes it.
• Emit .empty() for generated fixed repeated array validation instead of comparing .size() with zero.
• Regenerate checked smoke outputs and update handwritten smoke assertions to use .empty().

Root Cause

Generated fixed-array validation and several runtime helpers encoded non-empty checks as .size() != 0u or equivalent. That made emitted interfaces less idiomatic and inconsistent with container APIs that already expose .empty().

Validation

• uv run pytest (99 passed)
• git diff --check

Notes

• C++ smoke configure was attempted earlier in this workspace but the shell does not have clang-cl on PATH, so that build path was not run locally.

[anthonyprintup]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Another round soon, please!

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