perf: pre-reserve replace_rec_fn cache to avoid early rehashing

[Kha]

This PR pre-reserves 128 buckets in the replace_rec_fn cache when caching is enabled. The default std::unordered_map starts very small and rehashes several times during a typical traversal, and the resulting bucket-array churn dominated cache-miss stalls in the hot kernel traversal. Reserving up front shaves another ~4% off the wall-clock of leanchecker --fresh Init.Data.List.Lemmas on top of the previous try_emplace change, for a combined ~8% wall-clock improvement (with a small ~1.5% increase in retired instructions that is more than offset by the improved IPC).

[Kha]

!bench

[leanprover-radar]

Benchmark results for 1f4b906 against 82bb27f are in. Significant changes detected! @Kha

• 🟥 build//instructions: +17.5G (+0.15%)

Medium changes (3✅, 3🟥)

• 🟥 elab/big_match//instructions: +113.5M (+0.98%)
• 🟥 elab/cbv_divisors//instructions: +422.9M (+0.79%)
• 🟥 elab/cbv_leroy//instructions: +409.4M (+0.87%)
• ✅ elab/grind_ring_5//instructions: -103.9M (-1.17%)
• ✅ misc/leanchecker --fresh Init//task-clock: -1s (-5.50%)
• ✅ misc/leanchecker --fresh Init//wall-clock: -1s (-5.50%)

Small changes (7✅, 69🟥)

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

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Mathlib CI can not be attempted yet, as the nightly-testing-2026-04-09 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Mathlib CI should run now. You can force Mathlib CI using the force-mathlib-ci label. (2026-04-12 17:00:30)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2026-04-09 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2026-04-12 17:00:32)