fix(snapshot): preserve counters on resume

[chaliy]

Motivation

• Prevent fresh-instance snapshot resume from resetting session-level budgets when snapshots (including keyed/HMAC-protected ones) contain nonzero session counters.
• Keep protection against tampered/untrusted snapshot bytes lowering live counters while allowing authenticated snapshots to carry forward consumed budget.

Description

• Merge serialized session counters during restore by calling restore_session_counters from restore_snapshot_inner after validated shell/VFS restore so counters are applied atomically with state.
• Change Interpreter::restore_session_counters to be monotonic (use max) so restoring never reduces already-consumed session_commands or session_exec_calls.
• Update public Bash::restore_session_counters docstring to reflect monotonic merge semantics.
• Add integration tests covering keyed (HMAC) snapshot resume: keyed_snapshot_restore_carries_session_exec_budget_forward and from_snapshot_keyed_restores_session_counters to assert authenticated resumes carry counters forward.

Testing

• Ran the integration snapshot test binary: cargo test -p bashkit --test integration snapshot_tests:: -- --nocapture and all snapshot-related tests passed (40 passed, 0 failed).
• Ran cargo fmt --all -- --check to ensure formatting is clean and it passed.
• Verified the newly added snapshot integration tests execute and succeed as part of the snapshot test suite.

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Codex Task

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bashkit	d3db6d8	Commit Preview URL	Jun 02 2026, 09:33 AM