RUE-1208: splice borrow accessors at CFG threshold

[steveklabnik]

Summary

• preserve borrow-accessor calls as borrowed places through AIR and CFG, then mandatorily splice exact accessor CFG dependencies before optimization and code generation
• relocate accessor-local types, symbols, strings, spans, warnings, and destructor dependencies into each caller-owned domain while keeping raw callee keys shareable
• remove the epoch-engine eligibility fallback and keep accessor bodies out of the ABI/codegen-unit set
• add provider-path specification, UI, CLI, projected/nested/cycle, dual-backend, and exact warm-edit invalidation coverage

Validation

• scripts/rue quick
• /opt/homebrew/bin/bash ./clippy.sh
• RUE_TEST_TIER=premerge ./test.sh
• scripts/rue spec borrow-accessors
• scripts/rue ui borrow-accessors
• scripts/rue cli borrow_accessors

Fixes RUE-1208.
Fixes RUE-662.