docs: propose precise sandbox lifecycle states

[AiRanthem]

What changed

Adds the technical proposal for replacing the derived five-state Sandbox model with an eleven-state lifecycle through three ordered changes:

1. define and exhaustively verify the pure lifecycle model and in-place failure stages;
2. activate CR-based lookup and lifecycle routing on the approved short-ID Projector/Store/Repairer foundation;
3. atomically migrate consumers and add public E2B projection, operation gates, and reasoned Sandbox 404s.

Why

The current dead state conflates completion, failure, controller progress, and temporary unavailability. This can produce false 404s, premature route deletion, and duplicated CRD-condition interpretation in sandbox-manager.

Review focus

• lifecycle precedence and pre/update/post-mutation failure evidence;
• strict ownership boundaries with the short-ID design;
• route disposition and conditional peer deletion;
• SandboxSet/accounting semantics and the atomic consumer cutover;
• E2B public-state compatibility and the six 404 reasons.

Impact

Documentation only. No code, CRD schema, generated artifact, or public state enum changes are included in this PR.

Validation

• Companion OpenSpec source on feature/extend-states-260715: 3/3 changes pass strict validation.
• git diff --check passes.
• Proposal branch contains exactly one new document.

[kruise-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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Details

Needs approval from an approver in each of these files:

• docs/OWNERS

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.05%. Comparing base (dee78fb) to head (e2d6ea9).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #653      +/-   ##
==========================================
- Coverage   80.06%   80.05%   -0.02%     
==========================================
  Files         229      229              
  Lines       17861    17862       +1     
==========================================
- Hits        14300    14299       -1     
- Misses       2976     2978       +2     
  Partials      585      585              

Flag	Coverage Δ
unittests	80.05% <ø> (-0.02%)	⬇️

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