feat: respect user provided borrow annotations

[hargoniX]

• preserve in early pipeline
• first attempt

[hargoniX]

!bench

[leanprover-radar]

Benchmark results for d72df3e against 68ea28c are in! @hargoniX

• 🟥 build//instructions: +2.1G (+0.02%)

Small changes (2🟥)

• 🟥 build/module/Lean.Compiler.LCNF.InferBorrow//instructions: +395.0M (+6.04%) (reduced significance based on *//lines)
• 🟥 build/module/Lean.Compiler.LCNF.ToLCNF//instructions: +395.3M (+2.09%) (reduced significance based on *//lines)

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 68ea28c24f9ce74272a8f6aa88ad5bbc70f4034f --onto 333ab1c6f0ce11f33551d6a736054cb72812cad9. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-06 17:27:51)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 68ea28c24f9ce74272a8f6aa88ad5bbc70f4034f --onto cda84702e9b31165f1f83c657b532f36f34e0bd0. You can force reference manual CI using the force-manual-ci label. (2026-03-06 17:27:53)