feat: add difference on ExtDTreeMap/ExtTreeMap/TreeSet

[wkrozowski]

This PR adds a difference operation on ExtDTreeMap/ExtTreeMap/TreeSet and proves several lemmas about it.

Stacked on top of #11407

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 6eeb215e8f1986cd1628f8e02d6c69e337db034e --onto b0e6db322421ce368999f08870fd747e45242f86. You can force Mathlib CI using the force-mathlib-ci label. (2025-11-27 20:09:11)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 1ae680c5e2f3922ba0c6c257a105426fdab20999 --onto 5bd331e85d9d110a29fb3367dbb21854010ffcbd. You can force Mathlib CI using the force-mathlib-ci label. (2025-12-01 18:01:48)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase e548fa414cb74db21a31afe6c26cb0c51310f1db --onto 0173444d24df9253d7947cfe82519e4a6caffd3a. You can force Mathlib CI using the force-mathlib-ci label. (2025-12-04 12:15:46)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 42ded564bd27a6ac0f64c1daf4870f67472e1f59 --onto dd28f005889dd2fcca6fe0638133de561a655ad1. You can force Mathlib CI using the force-mathlib-ci label. (2025-12-04 18:45:24)

[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 6eeb215e8f1986cd1628f8e02d6c69e337db034e --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-11-27 20:09:12)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 1ae680c5e2f3922ba0c6c257a105426fdab20999 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-12-01 18:01:49)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase e548fa414cb74db21a31afe6c26cb0c51310f1db --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-12-04 12:15:48)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 42ded564bd27a6ac0f64c1daf4870f67472e1f59 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-12-04 18:45:25)

[wkrozowski]

@TwoFX Also merged the master here