feat: implement pvector<T> persistent vector with O(log n) access via AVL order-statistic tree (Issue #186)

[konard]

Implements pvector<T> — a persistent sequential container (vector) for the persistent address space (PAP) as requested in Issue #186.

Key features

• O(log n) random access via at(i) — uses an AVL order-statistic tree indexed by element position.
• O(1) size() — reads weight of root node (total subtree size).
• O(log n) push_back / pop_back / front / back — insert/remove rightmost node with AVL rebalancing.
• In-order iterator — traversal in element order via standard BST in-order walk.
• Nodes are not permanently locked — can be freed after removal.

Implementation

Each element is stored in a separate PAP block. The built-in TreeNode fields are used for the AVL tree:

• weight → total subtree size (self + all descendants). Enables O(log n) position lookup.
• left_offset / right_offset → left/right children (smaller/larger indices).
• parent_offset → parent node.
• avl_height → subtree height for AVL balance.

The _avl_find_by_index(p, i) function navigates O(log n) using subtree sizes:

• If i < left.weight → go left
• If i == left.weight → found
• If i > left.weight → go right with i -= left.weight + 1

Custom _rotate_left / _rotate_right and _rebalance_up maintain both height (AVL balance) and weight (subtree sizes) on every structural change.

Changed files

• include/pmm/pvector.h — New AVL order-statistic tree implementation (replaces linked-list)
• include/pmm/persist_memory_manager.h — Added Mgr::pvector<T> type alias
• tests/test_issue186_pvector.cpp — 14 test cases (added I186-N: AVL invariant verification)
• tests/CMakeLists.txt — Added test registration
• docs/api_reference.md — API documentation
• README.md — Updated with pvector section
• scripts/generate-single-headers.sh — Include pvector in single-header generation
• single_include/pmm/pmm.h — Regenerated single-header
• changelog.d/20260311_120000_pvector.md — Changelog fragment

Test plan

All 53 existing tests pass, plus 14 new pvector tests:

• push_back, at, size, empty, front, back, pop_back, clear, reset, iterators
• Block locking verification
• Stress test (100 elements with indexed access)
• Struct type support
• I186-N: AVL tree invariants — verifies weight fields are correct at every node, balance factor |bf|≤1, and tree height is O(log n)

Fixes #186

[konard]

🤖 Solution Draft Log

This log file contains the complete execution trace of the AI solution draft process.

💰 Cost estimation:

• Public pricing estimate: $8.969752
• Calculated by Anthropic: $5.589500 USD
• Difference: $-3.380252 (-37.69%)
  📎 Log file uploaded as Gist (2085KB)
  🔗 View complete solution draft log

---

Now working session is ended, feel free to review and add any feedback on the solution draft.

[konard]

✅ Ready to merge

This pull request is now ready to be merged:

• All CI checks have passed
• No merge conflicts
• No pending changes

---

Monitored by hive-mind with --auto-restart-until-mergeable flag

[netkeep80]

Используй встроенные avl-деревья в менеджер ПАП, что бы ускорить доступ к произвольному элементу вектора с O(n) до O(log n)

[konard]

🤖 AI Work Session Started

Starting automated work session at 2026-03-12T09:43:42.553Z

The PR has been converted to draft mode while work is in progress.

This comment marks the beginning of an AI work session. Please wait for the session to finish, and provide your feedback.

[konard]

AI Work Session Completed

Implemented the requested improvement: O(log n) random access via AVL order-statistic tree.

Changes made

The pvector now uses the built-in AVL tree infrastructure of the PAP manager instead of a linked list.

Key design:

• Each node's weight field stores its total subtree size (self + all descendants)
• at(i) navigates in O(log n) using subtree sizes: compare i against left.weight, go left/right, adjust index
• size() is now O(1) — reads weight of root
• push_back inserts as rightmost node, O(log n)
• pop_back removes rightmost node, O(log n)
• Custom _rotate_left/_rotate_right and _rebalance_up maintain both avl_height (for AVL balance) and weight (for subtree sizes) on every structural change

New test I186-N verifies AVL invariants at every node after 100 push_back + 30 pop_back operations:

• weight == 1 + left.weight + right.weight at every node
• Balance factor |bf| ≤ 1 at every node
• Tree height ≤ O(log n) bound

All 53 existing tests + 14 pvector tests pass. All CI checks pass.

[konard]

🤖 Solution Draft Log

This log file contains the complete execution trace of the AI solution draft process.

💰 Cost estimation:

• Public pricing estimate: $3.074593
• Calculated by Anthropic: $2.391127 USD
• Difference: $-0.683466 (-22.23%)
  📎 Log file uploaded as Gist (1571KB)
  🔗 View complete solution draft log

---

Now working session is ended, feel free to review and add any feedback on the solution draft.

[konard]

✅ Ready to merge

This pull request is now ready to be merged:

• All CI checks have passed
• No merge conflicts
• No pending changes

---

Monitored by hive-mind with --auto-restart-until-mergeable flag