feat: weak_specialize annotations

[hargoniX]

This PR introduces the weak_specialize attribute. Unlike the nospecialize attribute it does not
block specialization for parameters marked with this type completely. Instead, weak_specialize
parameters are only specialized for if another parameter provokes specialization. If no such
parameter exists, they are treated like nospecialize.

[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 0975b7136a8f8a7a1e224881d7c429376fae2026 --onto 4786e082dca22873d14d2a5b9b7c8843380c6e78. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-26 17:56:53)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 144db355ea259b0873b2f3d78795ffbd0994c787 --onto 4786e082dca22873d14d2a5b9b7c8843380c6e78. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-26 22:40:11)

[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 0975b7136a8f8a7a1e224881d7c429376fae2026 --onto cfa8c5a036d6990635c6ec50b02d0e806995cec3. You can force reference manual CI using the force-manual-ci label. (2026-03-26 17:56:54)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 144db355ea259b0873b2f3d78795ffbd0994c787 --onto cfa8c5a036d6990635c6ec50b02d0e806995cec3. You can force reference manual CI using the force-manual-ci label. (2026-03-26 22:40:13)