Split acceptance tests into focused modules

[yvette-carlisle]

Summary

• Split docs extension search/filter acceptance cases into a dedicated submodule.
• Split chunk search acceptance coverage into payload-level, relation-context, and filter-impact modules.
• Split graph ingestion single-predicate supersession coverage into its own module with shared setup helpers.

Size

• 8 files changed, 1647 insertions, 1528 deletions (3175 changed lines).

Validation

• RUSTC=/run/current-system/sw/bin/rustc cargo make fmt-rust && RUSTC=/run/current-system/sw/bin/rustc cargo make check
• git diff --cached --check