Preserve an unset caller IFS when loading provider facts #604

Goal

Preserve the caller's IFS state when provider facts are loaded in the current shell, including when IFS is intentionally unset under set -u.

Problem

The provider-facts wrapper merged in #603 snapshots IFS with local _acq_saved_ifs="$IFS". Under set -u, expanding an intentionally unset IFS exits before provider facts or fallback handling can run.

Scope

• Replace manual IFS snapshot/restore with the smallest Bash 3.2-safe local scoping mechanism.
• Preserve caller states where IFS is unset, empty, default, or custom.
• Preserve provider assignment propagation and all loader return/fallback behavior.
• Add focused regressions under set -u.

Acceptance criteria

• An unset caller IFS remains unset after successful load and every handled fallback/error result.
• Empty, default, and custom caller IFS values remain unchanged.
• Valid provider assignments remain visible to the caller.
• Return codes and fallback behavior remain unchanged.
• Focused and full Bash 3.2 checks pass.

Relationships

Parent: #600.
Follow-up from the non-blocking review note on #603.
Related epic: GSA-TTS/agentic-coding-patterns#357.

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