release/lib_release_params.sh: release_state_root silently falls back to the tree root when git fails — release state can split across two directories #3684

Found in the #3682 sibling sweep (0.69 batch #3669).

scripts/release/lib_release_params.sh:39, release_state_root():
common=$(git -C "$root" rev-parse --path-format=absolute --git-common-dir 2>/dev/null) || common="". When git fails, the state root silently becomes the tree itself instead of the main checkout. From a worktree, the release train's AP (<state_root>/target/release-train/<tag>) then points at a different directory than the one the rest of the train uses, and files such as cleanroom-run-id are written in one place and looked for in another. It fails later (publish_strict.sh dies on a missing run id), but far from the cause and with no mention of git.

It runs on hosts (the autopilot), not in the docker runners, so it is lower risk than #3682. It is the same "a git failure becomes a plausible answer" class.

done_when

• A git failure in release_state_root is an error naming git (rc 2), not a fallback. The fallback to $root applies only when git answered and the common dir is not */.git.
• A case row with a PATH git stub that exits 128 gives rc 2 naming git.

Refs #3682 #3669 #3655