Prove DstLayout padding and field extension

[joshlf]

Relate the production pad_to_align and extend methods to mathematical
layout operations. Packing limits placement alignment while retaining the
field's complete size, including its internal padding.

Use mathematical clauses for the operation specs and prove that they
retain the independent guarantees about alignment, normalized payload,
physical fields and padding flags. Keep representation and fit conditions
explicit.

Add total Rust harnesses comparing every stored field and both padding
branches with independent remainder-based checked arithmetic. The resulting
proofs establish the conditional layout behavior without requiring review
of intermediate Lean proof steps.

Agent-authored-by: AI agent acting on joshlf's behalf

---

• 　  Prove record construction against independent Rust references #3846
• 　  Prove selected cast plans preserve size without metadata overflow #3845
• 　  Prove CastPlan selection and unchecked metadata arithmetic #3844
• 　  Prove cast validation and exact-size metadata inference #3843
• 　  Prove allocation-size preparation and tail containment #3842
• 　  Prove CastUnsized's production layout acceptance gate #3841
• 　  Prove the numerical bounds used by split operations #3840
• 　  Prove advancement and padding-sequence decisions #3839
• 👉 Prove DstLayout padding and field extension #3838
• 　  Prove unit and usize PointerMetadata operations #3837
• 　  Prove trailing sizes, padding and capacity including overflow #3836
• 　  Prove primitive DstLayout constructors #3835
• 　  Add an independent Rust reference for nested DST sizes #3834
• 　  Specify rounding encodings as alignment and phase #3833
• 　  Relate recursive record layout to normalized size formulas #3832
• 　  Prove normalized trailing-size and capacity formulas #3831
• 　  Reuse Aeneas rules for views and indexed loops #3830
• 　  Prove least padding and greatest aligned predecessors #3829
• 　  Test total and partial Aeneas contract semantics #3828
• 　  Verify Boolean and NonZeroUsize candidate predicates #3852
• 　  Verify the byte writers used by IntoBytes #3851
• 　  Prove U16 and U32 byte-order reads, writes and updates #3827
• 　  Prove guarded Boolean conversion and byte copying #3849
• 　  Reject unsupported execution effects before Aeneas lowering #3848
• 　  Check Rust specs against golden and live Aeneas models #3826
• 　  Separate cast plans from pointer reconstruction #3825
• 　  Move cast_from into layout/cast_from.rs #3824
• 　  Share Rust layout and byte models in anneal/lean #3823
• 　  Update Anneal to Aeneas ca282ec and Lean 4.31 #3822

Latest Update: v8 — Compare vs v7

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v5				vs v4	vs v3	vs v2	vs v1	vs Base
v4					vs v3	vs v2	vs v1	vs Base
v3						vs v2	vs v1	vs Base
v2							vs v1	vs Base
v1								vs Base

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[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-08T10:26:02.002770Z	3401b31	PR opened
🔒 Security Review	✅ Completed 2026-10-08T10:26:01.950965Z	3401b31	PR opened

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 119 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.09%. Comparing base (6112ae4) to head (3401b31).

Files with missing lines	Patch %	Lines
zerocopy/src/layout/composition_checks.rs	0.00%	119 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                          @@
##           Gasqy37pxeoegrksputrmrts2btqezgvh    #3838      +/-   ##
=====================================================================
- Coverage                              57.89%   56.09%   -1.80%     
=====================================================================
  Files                                     24       25       +1     
  Lines                                   3947     4066     +119     
=====================================================================
- Hits                                    2285     2281       -4     
- Misses                                  1662     1785     +123     

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