Add scripts/rolling-bootstrap.sh to rebuild the self-hosting chain from
the last release binary up to the checked-out revision. A released binary
can only compile source up to its own feature set, so between releases it
can no longer cold-bootstrap master directly — the moment a feature is
taught to the parser in one commit and used in the runtime/compiler in a
later commit, the gap opens. The script replays the chain commit-by-commit
(each step built by the previous step's binary), carrying the binary
forward, and installs the result as releases/v<version>-pre/.
It runs in a throwaway git worktree, so the live tree (including
uncommitted changes) is untouched; it passes QBE= and BLAISE= into the
runtime Makefile and uses the main repo's built QBE; and it smoke-tests
each step. A failed step names the exact commit (BOOTSTRAP_BROKEN),
making it usable as a CI check that the two-step discipline holds.
Document the prerequisite (place the last release binary under
releases/vX.Y.Z/, which is gitignored) and usage in scripts/BOOTSTRAP.adoc,
with a pointer from README.adoc.
