Distinguish vulnerability, chosen openness, and imposed exposure

[raelovejoy]

Why

The existing Finitude, Love, and Continuance essay already connects wonder to ordinary care. The Existential Euphoria discussion sharpened a nearby human-scale distinction: being affectable is part of life, but vulnerability should not be confused with coerced exposure or manufactured precarity.

Changes

• add a provisional section to src/pages/core/finitude-love-continuance.md;
• distinguish existential vulnerability, chosen openness, and imposed vulnerability;
• frame boundaries and refusal as possible supports for continued connection;
• preserve distance, privacy, caution, numbness, and withdrawal as potentially protective responses;
• add vulnerability and boundaries to the related threads.

No one is required to interpret loss or vulnerability as liberating.

No merge requested; review first.