Document bootstrap chain ordering with worked example

Add a "Bootstrap chain ordering" section to docs/usage.md explaining when --bootstrap-strategy=topo helps. Walks through a small merge graph showing why first-parent fails on a side-branch that doesn't fit and how topo places sub-pack boundaries inside the side branch instead. Also restates the temp-ref non-fast- forward server requirement that's currently only documented in the Go doc-comment, so users hitting the flag from the CLI don't have to dig into the package source to find it.