feat: make the borrow inference explain itself

[hargoniX]

This PR adds tracing to the borrow inference to explain to the user why it got to its conclusions.

[hargoniX]

!bench

[leanprover-radar]

Benchmark results for 65fcbae against 8d42ad4 are in! @hargoniX

• 🟥 build//instructions: +996.1M (+0.01%)

Small changes (3🟥)

• 🟥 build/module/Lean.Compiler.LCNF.InferBorrow//instructions: +1.0G (+18.08%) (reduced significance based on *//lines)
• 🟥 build/module/Lean.Compiler.LCNF.Main//instructions: +73.1M (+1.58%) (reduced significance based on *//lines)
• 🟥 build/module/Lean.Compiler.Options//instructions: +19.7M (+3.60%)

[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 8d42ad47968312034f9a8acb7cd8d650d270cada --onto 333ab1c6f0ce11f33551d6a736054cb72812cad9. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-05 14:03:15)

[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 8d42ad47968312034f9a8acb7cd8d650d270cada --onto cda84702e9b31165f1f83c657b532f36f34e0bd0. You can force reference manual CI using the force-manual-ci label. (2026-03-05 14:03:16)