bootstrap: stream commit-graph fetch to bound planning memory

[nodo]

Replace the per-branch memory.NewStorage() planning fetch with a streaming extractor that pulls only (commit -> parent hashes) tuples from the tree:0-filtered pack.

Previously, bootstrap planning materialized the entire commit set (~1.4M decoded commits for linux, ~5 GiB) just to read each commit's ParentHashes. The new path reads the pack incrementally, extracting parents from each commit as it arrives and discarding the bytes; a bounded LRU keeps recent objects in memory only long enough to resolve deltas. Planning then walks the resulting parents map directly.

Measured against replicate --all-refs https://github.com/torvalds/linux.git into a local git-http-backend target:

                            before     after     delta
  Peak Go heap (inuse)      5.42 GiB   1.47 GiB  -73%
  Peak OS RSS               5.69 GiB   1.63 GiB  -71%
  Wall time (4 GiB batches) 32m 04s    19m 09s   -40%
  Refs pushed               2862/2862  2862/2862 same

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Note

Medium Risk
Moderate risk: introduces new low-memory pack parsing logic (including temp-file spill and delta-base caching) and refactors v2 fetch envelope handling, which could affect correctness/performance on unusual pack/delta layouts or constrained disks.

Overview
Adds a streaming ExtractCommitParents path that parses a tree:0 packfile and returns only (commit -> parent hashes), using a bounded LRU cache for delta resolution and spilling non-seekable inputs to a temp pack file to keep go-git in low-memory mode.

Introduces RefService.FetchCommitParents and refactors v2 fetch parsing into consumeV2FetchPack, allowing callers to either store the pack (UpdateObjectStorage) or consume the demuxed pack stream.

Updates batched bootstrap planning to fetch only commit-parent metadata (instead of materializing a full in-memory object store) and adds new planner walkers FirstParentChainFromParents/TopoChainFromParents plus tests to validate they match the storer-based traversal.

^{Reviewed by Cursor Bugbot for commit ede27fd. Configure here.}