[08/08] add benchmarks and graph internals documentation for migrated pipeline

[mjc]

Stacked on mjc/gitoxide-spike-07-draw-status.

Why this PR exists:

• This is the documentation and benchmark-fixture maintenance layer at the top of the stack.
• Earlier PRs changed the graph pipeline, lane-cap representation, graph history storage, and gix-backed metadata shapes.
• This PR updates GRAPH.MD so the documented graph model matches the final migrated implementation.
• It also fixes benchmark fixtures so they construct graph/worktree data with the current migrated types.

What changed:

• Updates GRAPH.MD to describe the current graph pipeline.
• Replaces the older compressed-parent explanation with the current flattened lane-cap model.
• Documents that flattened lanes are represented directly on Chunk through is_flattened.
• Removes references to a separate compressed_parents side list.
• Updates delta documentation to match the current delta ops:
  • Insert
  • Remove
  • Replace
  • Truncate
  • ReplaceAndTruncate
• Updates graph history documentation from GraphSnapshot { lanes, compressed_parents } to lane snapshots.
• Updates renderer notes so merge closeouts and dotted cap-lane rendering are described in terms of flattened cap lanes.
• Updates the module map and vocabulary to match the migrated graph/buffer/rendering implementation.
• Fixes benchmark fixtures for the current data model:
  • converts benchmark worktree heads from git2::Oid to gix::ObjectId
  • updates GraphRow fixture construction for the current fields and collection types

Algorithm difference:

• No runtime behavior change is intended.
• The documentation now describes the algorithm that already exists in the previous stack layers.
• The benchmark fixture updates are type/shape fixes for the migrated graph and worktree metadata structures.

Performance story:

• This PR does not add new benchmark results by itself.
• It keeps benchmark fixtures aligned with the migrated graph data model so existing benchmark targets can continue to exercise the current pipeline.
• Any performance claims should come from running the benchmark suite, not from this PR alone.

Correctness gate:

• GRAPH.MD should match the actual graph implementation after the migration stack.
• The docs should not describe removed structures such as compressed_parents.
• Benchmark fixtures should compile against the current graph, worktree, and row metadata types.
• No app, graph, query, action, or draw behavior should change in this layer.

Benchmark Results for the whole stack

Command shape: hyperfine --warmup 5 './target/release/guitar /home/mjc/src/linux --exit-when-graph-complete'.

CPU: M1 MacBook Pro 16GB

Ref	Total	vs main	User	System	CPU
main	2:37.28	1.00x	118.00s	7.88s	80%
01-gix-boundary	1:43.99	1.51x	97.85s	2.33s	96%
02-gix-oids	1:14.26	2.12x	65.41s	2.17s	91%
03-graph-walker	5.816s	27.05x	1.82s	0.74s	43%
04-queries-readonly	3.812s	41.26x	1.47s	0.42s	49%
05-actions-network	1.993s	78.91x	1.47s	0.26s	87%
06-app-integration	1.826s	86.13x	1.51s	0.10s	88%
06-app-integration	1.879s	83.70x	1.44s	0.11s	82%
07-draw-status	1.618s	97.21x	1.36s	0.08s	89%

CPU: 5950x Linux/ZFS

Ref	Total	vs main	User	System	CPU
main	1:39.24	1.00x	96.16s	1.72s	98%
01-gix-boundary	1:46.41	0.93x	101.28s	3.82s	98%
02-gix-oids	1:10.85	1.40x	69.07s	1.78s	100%
03-graph-walker	2.094s	47.39x	1.59s	0.55s	102%
04-queries-readonly	1.856s	53.47x	1.58s	0.46s	110%
05-actions-network	1.862s	53.30x	1.60s	0.49s	112%
06-app-integration	1.639s	60.55x	1.40s	0.22s	98%
07-draw-status	1.632s	60.81x	1.34s	0.18s	93%
08-bench-docs	1.629s	60.92x	1.36s	0.19s	94%

Memory: 5950x Linux/ZFS

Ref	Peak heap	Peak RSS
main	1.26G	2.81G
01-gix-boundary	1.26G	2.82G
02-gix-oids	1.38G	2.63G
03-graph-walker	455.13M	1.15G
04-queries-readonly	436.12M	977.18M
05-actions-network	436.12M	978.61M
06-app-integration	419.89M	853.30M
07-draw-status	419.89M	916.59M
08-bench-docs	419.89M	988.71M

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