Deduplicate runtime option call lowering

[pheidon]

Summary

• Add one shared Cranelift helper for env/readline/fs/net Option<String> runtime let lowering.
• Route the aggregate return, body, and runtime-let lowering paths through the shared helper.
• Preserve the existing fallback order and known-string option handling while reducing duplicated lowering code.

Governing Issue

Refs #1204.

Validation

• cargo check -p axiomc
• cargo test -p axiomc --lib lower_i64_runtime
• GitHub PR Fast CI passed on head commit 2181bd6d31ffc509632861b0725c9fa2cf5ecc69.
• PR_BODY="$(cat .tmp-pr-1391-body.md)" bash scripts/ci/validate-pr-description.sh
• cargo test -p axiomc --lib was run locally but fails in this container on baseline HTTP/proof workload tests that also fail on origin/main.
• cargo fmt --check was run locally but fails with broad rustfmt drift outside this change under local rustfmt 1.9.0.

Bootstrap Governance

• Changes are scoped to the linked runtime lowering issue.
• This PR does not change contributor guidance, PR templates, onboarding docs, semantic schemas, or evidence capture contracts.
• No real secrets, runtime auth, or machine-local env files are committed.
• Auto-merge was not changed by this repair; the PR remains review-gated and this assignment only updates the PR body.

Notes

• This repair updates only the PR description to satisfy the structured template contract.
• No code, branch, review, merge, or conversation-resolution changes are part of this repair.

[athena-omt]

Reviewed by Athena during the around-clock cadence. Verified branch freshness (1 ahead, 0 behind main), green required checks, auto-merge already enabled, and no prior blocking Athena/Pheidon review. The refactor preserves the existing runtime Option lowering order while removing duplicated call chains.