[improvement][feat] Deterministic IPv6 /112 PodCIDR via :0:c::/112; update tests/docs

[komer3]

• Add computeStableIPv6PodCIDR() to derive a stable /112 from a node’s /64 using mnemonic subprefix :0:c::/112.
• allocateIPv6CIDR(): prefer stable subprefix when mask=112 and base is /64; otherwise fall back to start-of-range masking.
• Update tests in cloud/nodeipam/ipam/cloud_allocator_test.go to expect 2300:5800:2:1:0:c::/112.
• Document behavior and correct example flag to --node-cidr-mask-size-ipv6=112 in docs/configuration/nodeipam.md.
• Add/adjust debug logs for clarity; no change to IPv4 behavior.

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• Have you removed all sensitive information, including but not limited to access keys and passwords?
• Have you checked to ensure there aren't other open or closed Pull Requests for the same bug/feature/question?

Pull Request Guidelines:

1. Does your submission pass tests?
2. Have you added tests?
3. Are you addressing a single feature in this PR?
4. Are your commits atomic, addressing one change per commit?
5. Are you following the conventions of the language?
6. Have you saved your large formatting changes for a different PR, so we can focus on your work?
7. Have you explained your rationale for why this feature is needed?
8. Have you linked your PR to an open issue

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.88%. Comparing base (df34350) to head (590d969).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
cloud/nodeipam/ipam/cloud_allocator.go	80.00%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #439      +/-   ##
==========================================
+ Coverage   71.87%   71.88%   +0.01%     
==========================================
  Files          19       19              
  Lines        3598     3625      +27     
==========================================
+ Hits         2586     2606      +20     
- Misses        773      779       +6     
- Partials      239      240       +1     

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[Copilot]

Pull Request Overview

This PR introduces deterministic IPv6 PodCIDR allocation using a stable mnemonic subprefix :0:c::/112 within a node's /64 IPv6 range. The change ensures consistent PodCIDR assignment across restarts while maintaining backward compatibility.

• Implements computeStableIPv6PodCIDR() function to derive stable /112 subnets from /64 base ranges
• Updates allocation logic to prefer stable subprefix when conditions are met, falling back to original behavior otherwise
• Corrects documentation example and updates test expectations to reflect the new deterministic behavior

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
cloud/nodeipam/ipam/cloud_allocator.go	Adds stable IPv6 PodCIDR computation logic and updates allocation method
cloud/nodeipam/ipam/cloud_allocator_test.go	Updates test expectations to use new deterministic IPv6 addresses
docs/configuration/nodeipam.md	Documents new behavior and corrects configuration example

[AshleyDumaine]

LGTM

[Copilot]

Pull Request Overview

This PR implements deterministic IPv6 PodCIDR allocation using a stable mnemonic subprefix :0:c::/112 to derive /112 subnets from node /64 IPv6 ranges. This ensures consistent IPv6 pod networking across node restarts and deployments.

• Adds getIPv6PodCIDR() function to compute stable /112 PodCIDRs using mnemonic subprefix :0:c::/112
• Updates allocateIPv6CIDR() to prefer stable allocation when conditions are met, with fallback to original behavior
• Updates all test expectations and documentation to reflect the new stable IPv6 allocation behavior

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
cloud/nodeipam/ipam/cloud_allocator.go	Core implementation adding stable IPv6 PodCIDR computation with mnemonic subprefix
cloud/nodeipam/ipam/cloud_allocator_test.go	New test for stable IPv6 function and updated expected values across existing tests
docs/configuration/nodeipam.md	Documentation updates explaining stable IPv6 behavior and correcting example flag value