perf: xref-only bootstrap and lazy fast paths across count, split, and merge (#6)

* wip(supremacy): baseline — bench artifacts + no-clone walk prototype

* wip(supremacy): xref-only-bootstrap

* wip(supremacy): trusted-count-fast-path

* wip(supremacy): normal-object-arc-cache

* wip(supremacy): lean-split-outputs

* wip(supremacy): final shootout + report

* perf(split): skip resource pruning when an output covers the whole document

* perf(split): lazy source with early-stopped page walk for bounded ranges

* docs: add benchmark chart SVG and generator script

* docs: structure benchmark SVG footer into labeled columns

* docs: move benchmark results into the README and ship the fixtures generator

* style: cargo fmt