fix: make array mutator methods mutate the receiver in place

[jtippett]

Summary

push, pop, shift, unshift, splice, reverse, and fill computed their return value against a clone and never mutated the receiver — arr.push(x) didn't persist. Another silent-wrong-answer case on a documented-supported surface (array methods).

Changes

• Mutator methods mutate in place and write back to the receiver variable, consistent with the value-typed SetIndex/SetProperty store-back pattern described in AGENTS.md.
• Regression tests in tests/objects_arrays.rs.

Test plan

• Unit tests pass (cargo test)
• make lint clean
• CI passes

Related issues

None open. We run this patch in production via ex_zapcode.

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📥 Commits

Reviewing files that changed from the base of the PR and between ebc06fd and 9159f2a.

📒 Files selected for processing (3)

• crates/zapcode-core/src/vm/builtins.rs
• crates/zapcode-core/src/vm/mod.rs
• crates/zapcode-core/tests/objects_arrays.rs

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