Let ObjectMappers callers select the Jackson buffer recycler pool

[parker89]

Adds ObjectMappers.RecyclerPoolType { THREAD_LOCAL, SHARED } and RecyclerPoolType-accepting overloads on the factory/mapper methods. Default is unchanged (THREAD_LOCAL = Jackson's default), so this is a pure, opt-in API addition.

Motivation: Jackson's default BufferRecycler pool is thread-local, which virtual threads defeat — each short-lived vthread reallocates its parse/generate buffers instead of recycling them (observed as ~900 MB/s of extra byte[]/char[] churn via BufferRecycler.balloc/calloc in a vthread service). Callers that run on virtual threads can now pass SHARED to get a thread-identity-independent pool.

The selection is an explicit caller decision rather than runtime thread detection: a caller that knows its execution model (e.g. witchcraft, which knows whether the request pool is virtual) picks the pool. Wiring it through the server (conjure-undertow Encodings) and, if desired, clients (dialogue-serde) is left to follow-up PRs in those repos.

[changelog-app]

Generate changelog in changelog/@unreleased

Type (Select exactly one)

• Feature (Adding new functionality)
• Improvement (Improving existing functionality)
• Fix (Fixing an issue with existing functionality)
• Break (Creating a new major version by breaking public APIs)
• Deprecation (Removing functionality in a non-breaking way)
• Migration (Automatically moving data/functionality to a new system)

Description

Route Jackson buffer recyclers to a shared pool on virtual threads

Check the box to generate changelog(s)

• Generate changelog entry