fix(interpreter): gate allexport env updates

[chaliy]

Motivation

• Prevent set -a from persisting rejected assignments into the persistent environment and thereby bypassing the per-instance variable memory limits (TM-DOS-060).
• Ensure env mirroring only occurs when the interpreter actually accepts the variable insert under the configured MemoryLimits.
• Also prevent local updates under allexport from mutating env before budget checks succeed.

Description

• Change insert_variable_checked to return a bool success indicator so callers can know whether the allocation was accepted while preserving silent rejection behavior.
• In set_variable, run the memory budget check_variable_insert/record_variable_insert for existing local bindings before mutating the local frame or calling insert_env_checked under ALLEXPORT.
• For global assignments with ALLEXPORT, only call insert_env_checked after insert_variable_checked returns true so rejected global inserts are not mirrored into self.env.
• Add two regression unit tests test_allexport_rejected_global_update_does_not_mutate_env and test_allexport_rejected_local_update_does_not_mutate_env to ensure rejected inserts leave both shell variables and env unchanged.

Testing

• Ran cargo test -p bashkit interpreter::tests::test_allexport_rejected --lib and the new regression tests passed.
• Ran cargo fmt --check && cargo test -p bashkit allexport --lib and the allexport test group passed.
• All added unit tests (test_allexport_rejected_*) and the existing test_allexport_respects_env_memory_limits passed.

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Codex Task

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bashkit	f3dee9c	Commit Preview URL	Jun 02 2026, 09:24 AM