Honor clone freeze keywords for Config and SequenceSet

[OskarEichler]

Summary

Forward clone freeze keywords to Config internal data and honor them when copying SequenceSet runs. Thawed copies need mutable independent data; explicitly frozen copies need frozen internal data. Default frozen SequenceSet clones retain their shallow-copy optimization.

Reproduction

require 'net/imap'
original = Net::IMAP::SequenceSet['1:3']
copy = original.clone(freeze: false)
copy.delete(1)
p [original.to_s, copy.to_s, copy.frozen?] # ["1:3", "2:3", false]
config = Net::IMAP::Config.default.clone(freeze: false)
config.open_timeout = 12
p config.open_timeout # 12
# Before: both clone calls raise ArgumentError for the keyword argument.

Verification

• 212 focused checks; 78 failing expectations before, zero afterward. Config/SequenceSet cases cover mutable/frozen/default/empty/full originals, omitted freeze and nil/false/true, internal frozen state, mutation and original ownership. Existing 🐛 Fix Config#clone to clone internal data struct #303/♻️ Refactor SequenceSet#dup, #clone, and #replace #505 fix earlier copying behavior; open 🔧 Make Config.global Ractor shareable #595 proposes a broader global/Ractor config design and is not included.
• Existing rake test on this isolated branch: 1726 tests, 12592 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications, Ruby 4.0.6 via rbenv. The baseline also passes all 1,726 tests; assertion counts vary slightly across runs.
• Supplemental RuboCop Lint retains the same 48 existing findings. Ruby syntax and git diff --check pass. No repository tests, dependencies or workflows were added or modified; focused reproductions live outside the repository under the consumer's no-new-tests policy.
• Independent branch based on 6d2ef7a636a1e2449187a83b06ac7a5baa54ead2; the runtime diff from released 0.6.6 is documentation-only before this patch.

Compatibility and limits

No new public API or Ruby minimum. Standard clone(freeze:) calls now work. Default clone and dup semantics remain. This patch does not make global configuration Ractor-shareable. Other Ruby/OS versions were not run locally. No production or external IMAP service was used. Local verification does not imply upstream CI approval or comprehensive behavioral coverage.

[nevans]

Thanks for the PRs! I'm not sure what "focused reproductions live outside the repository under the consumer's no-new-tests policy" means, but tests are generally appreciated. :)