Keep large ordered sequence indices as integers

[OskarEichler]

Summary

Return an integer position from find_ordered_index without applying sequence-number star conversion. Ordered entries may repeat, so their indices can reach 2**32 even though individual sequence numbers cannot. That integer currently becomes :*, breaking the index contract.

Reproduction

require 'net/imap'
set = Net::IMAP::SequenceSet['1:4294967294,1:2,4294967295']
p set.find_ordered_index(4294967295)
# before: :*; after: 4294967296
# Only three stored ranges are used; no huge array is expanded.

Verification

• 11 focused checks; two failing expectations before, zero afterward. Compact inputs exercise indices below/at/above 2**32, star entries, ordinary indices, absent numbers and the ordered_at inverse without enumerating large ranges.
• 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 signature change. The affected boundary now returns its documented Integer index instead of a star Symbol. Actual sequence-number star conversion remains unchanged. 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.