feat: introduce Context type for cancellation with context propagation

[algebraic-dev]

This PR adds the Context type for cancellation with context propagation. It works by storing a tree of forks of the main context, providing a way to control cancellation.

[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 856825a4d2741f2de6d2d21454c20b1ba78b671b --onto 5bd331e85d9d110a29fb3367dbb21854010ffcbd. You can force Mathlib CI using the force-mathlib-ci label. (2025-12-03 23:19:01)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 856825a4d2741f2de6d2d21454c20b1ba78b671b --onto dd28f005889dd2fcca6fe0638133de561a655ad1. You can force Mathlib CI using the force-mathlib-ci label. (2025-12-05 04:57:47)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase c9b8508f6b6868be1857d207edf9ac01d764f8d2 --onto 62f2f9229356039356c0469a46d8ecd77be88e2a. You can force Mathlib CI using the force-mathlib-ci label. (2025-12-09 00:49:34)

[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 856825a4d2741f2de6d2d21454c20b1ba78b671b --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-12-03 23:19:03)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase c9b8508f6b6868be1857d207edf9ac01d764f8d2 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-12-09 00:49:35)