feat: expire idle task pool threads after 5 seconds

[Kha]

This PR makes the task thread pool reclaim idle worker threads after 5 seconds of inactivity. Previously, pool threads were allocated on demand but never freed, which could waste significant memory given the new default 1GB stack size per thread.

Worker threads are now detached immediately after creation instead of being stored in a vector. Each idle thread uses a timed wait and exits if no work arrives within the timeout. The destructor waits on a counter instead of joining stored threads.

[Kha]

!bench

[leanprover-radar]

Benchmark results for d567797 against 6f98a76 are in. Significant changes detected! @Kha

• 🟥 build//instructions: +38.6G (+0.31%)

Large changes (2🟥)

• 🟥 compiled/channel//wall-clock: +3s (+117.07%)
• 🟥 misc/re-elab Init.Data.List.Basic//task-clock: +2s (+13.89%)

Medium changes (1✅, 7🟥)

• 🟥 build/module/Std.Data.DTreeMap.Internal.Lemmas//instructions: +1.5G (+0.69%)
• 🟥 compiled/channel//task-clock: +8s (+83.08%)
• ✅ interpreted/identifier_completion//instructions: -1.2G (-1.63%)
• 🟥 lake/inundation/build no-op//instructions: +245.5M (+4.24%)
• 🟥 misc/import Init.Data.List.Sublist//instructions: +129.6M (+1.10%)
• 🟥 misc/import Init.Prelude//instructions: +223.4M (+1.82%)
• 🟥 misc/import Std.Data.Internal.List.Associative//instructions: +868.8M (+1.10%)
• 🟥 misc/re-elab Init.Data.List.Basic//instructions: +6.1G (+3.89%)

Small changes (1✅, 301🟥)

Too many entries to display here. View the full report on radar instead.

[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 6f98a76d01470ff9aa1d0895cfd2827eaa24ec89 --onto e6df474dd9c3ad0e21771eaa808c53f66222216d. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-25 19:02:43)

[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 6f98a76d01470ff9aa1d0895cfd2827eaa24ec89 --onto cfa8c5a036d6990635c6ec50b02d0e806995cec3. You can force reference manual CI using the force-manual-ci label. (2026-03-25 19:02:45)