[tae898]

Closes #190

The "Rebuild the graph" section of how-to/vectors.adoc gains three things:

• one sentence: the background rebuild starts on the first search after the threshold, so an index that only takes writes rebuilds when it is next searched;
• the build_graph_now() paragraph now says that searches on that index wait until it returns while inserts carry on, that Ctrl-C takes effect only when it returns, and that a process killed during the build leaves a consistent database;
• a short subsection, "Agents that write as they go": the default needs no setting; for a lower search tail, turn the automatic rebuild off and call build_graph_now() right before a step without searches (such as the call to the language model), with a table of what that bought and cost on each package.

Sources: the timing numbers come from the agent-memory study (timed with ab.py --noise, 12 clean pairs per package, both packages of 26.11.1.dev0); the table is generated from its score files and every figure was checked against them. The robustness sentences come from probes on both packages of the matched build (bindings dd45b53947, engine e353330c5c): searches waited for the whole build with the GIL released, inserts kept going, SIGKILL and SIGINT mid-build were followed by a clean CHECK DATABASE. The page carries no probe timings (only ab.py figures go into the docs).

Checks: pre-commit passes on the file; the Antora build runs in the docs workflow.

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