fix: @[nospecialize] is never template-like

[Kha]

This PR avoids false-positive error messages on specialization restrictions under the module system when the declaration is explicitly marked as not specializable. It could also provide some minor public size and rebuild savings.

[Kha]

!bench

[leanprover-radar]

Benchmark results for a7a4f89 against 8f80881 are in! @Kha

• ✅ build//instructions: -559.9M (-0.00%)

Medium changes (2✅, 2🟥)

• ✅ build/module/Lake.Build.Job.Monad//bytes .olean: -82kiB (-24.88%)
• 🟥 build/module/Lake.Build.Job.Monad//bytes .olean.private: +84kiB (+24.04%)
• ✅ build/module/Lake.Build.Job.Register//bytes .olean: -32kiB (-32.96%)
• 🟥 build/module/Lake.Build.Job.Register//bytes .olean.private: +32kiB (+36.53%)

[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 8f80881c2f7611c61d0228dac94b27582b44d5d0 --onto e7e3588c973226e85d79ed0f3154cfca79b61c6f. You can force Mathlib CI using the force-mathlib-ci label. (2026-02-23 20:35:50)

[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 8f80881c2f7611c61d0228dac94b27582b44d5d0 --onto 8038a8b8904f89ad9542c8eda11379f8f006eab1. You can force reference manual CI using the force-manual-ci label. (2026-02-23 20:35:52)