8376842: Reorganize ZipCoder hierarchies to handle UTF8 in the base classes

[eirbjo]

This PR reorganizes java.util.zip.ZipCoder class hierarchies to handle UTF-8 in the base class and move support for general charsets down to a subclass. This better reflects reality where most ZIP files are encoded using UTF-8.

The motivation can be summarized as:

• Supporting the common case in the base class and the exceptional case in a subclass seems more natural and intuitive for maintainers
• We avoid loading both classes for most JVM instances
• A small performance improvement is observed, probably caused by unlocking effecively-monomorphic JVM optimizations

See core-libs-dev discussion for details: https://mail.openjdk.org/pipermail/core-libs-dev/2026-January/158281.html

I have tried to not move methods around too much, making the side-by-side diff when ignoring whitespace easier to review. We can discuss further cleanups and reorganizaton if so desired.

It may be useful to review commits individually. The first commit fe4a627 moves some methods out the way for the second commit 2926398.

This PR also includes the similar change to the ZipFileSystem implementation of ZipCoder to keep these aligned. Could be pulled out to a separate PR if reviewers want to focus on a single area per PR.

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8376842: Reorganize ZipCoder hierarchies to handle UTF8 in the base classes (Enhancement - P4)

Reviewing

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$ git fetch https://git.openjdk.org/jdk.git pull/30640/head:pull/30640
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Update a local copy of the PR:
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$ git pull https://git.openjdk.org/jdk.git pull/30640/head

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$ git pr checkout 30640

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https://git.openjdk.org/jdk/pull/30640.diff

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