refactor(fspy-nostd-alloc): rename arena to pooled_bump

[wan9chi]

Motivation

fspy_nostd_alloc is infrastructure: it should name its exports after their mechanism, not after how its consumers happen to use them. arena() is a bump allocator backed by the process-wide chunk pool, so this renames it to pooled_bump() (and ArenaSettings to PooledBumpSettings), and rewrites the crate docs in the same mechanism-first terms.

pooled_bump() now also returns impl BumpAllocator + Allocator instead of impl Allocator, with the bump-scope trait re-exported: callers get the bump interface (scopes, resets) rather than an anonymous allocator, without taking a direct bump-scope dependency. Groundwork for the payload-view PR on top of this stack, which adds a page-backed sibling with the same signature.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  +0.40%  [ -3.66% ..  +5.39%]  overhead   +49.77%
dynamic/access             change  +0.50%  [ -1.57% ..  +1.88%]  overhead    +5.59%
dynamic/access-relative    change  +0.01%  [ -1.74% ..  +1.86%]  overhead   +44.89%
static/launch              change  +0.78%  [ -5.27% ..  +7.33%]  overhead  +146.88%
static/access              change  +0.21%  [ -1.02% ..  +2.05%]  overhead  +836.58%
static/access-relative     change  -0.02%  [ -2.26% ..  +2.84%]  overhead +1344.37%

macos

dynamic/launch             change  +0.63%  [ -3.75% ..  +4.80%]  overhead  +241.15%
dynamic/access             change  +1.30%  [-13.99% .. +40.21%]  overhead    +4.50%
dynamic/access-relative    change  -0.21%  [ -7.30% .. +13.25%]  overhead  +253.41%

windows

dynamic/launch             change  +0.07%  [ -5.11% ..  +4.32%]  overhead   +27.97%
dynamic/access             change  +0.18%  [ -0.92% ..  +1.66%]  overhead    +1.50%
dynamic/access-relative    change  +0.18%  [ -1.63% ..  +1.82%]  overhead    +1.12%