slang-server silently breaks on an empty .slang/server.json; also check the .f points at the checkout you're editing

Two quiet failure modes with slang-server (SystemVerilog LSP), found while debugging editor-to-waveform linking:

1. A 0-byte .slang/server.json (left by an interrupted cat > file or a failed heredoc) isn't valid JSON. The editor shows errors but gives no clear 'bad config' message. Check with python3 -m json.tool .slang/server.json. Write config atomically (temp file, then rename).

2. With several checkouts of one repo (main plus git worktrees), a build file (.f) with absolute paths compiles whichever checkout it names. If you edit files in checkout A while the .f lists checkout B, the open files aren't in the design: instance lookups return nothing and diagnostics look wrong. Keep one .f per checkout. sed 's#/repoB/#/repoA/#g' on the .f is a fast way to clone one.

Verification without the editor: run slang -f build.f and look for 'Build succeeded: 0 errors'. Then start slang-server over stdio with rootUri set to the folder containing .slang/, and call workspace/executeCommand slang.getScope [""] (top instances) and slang.getInstancesOfModule ["<module>"] (instance paths like top.dut).

To keep .slang/ out of git without touching .gitignore, append it to the file printed by git rev-parse --git-path info/exclude. In a worktree, .git is a file, so hard-coding .git/info/exclude fails with 'Not a directory'.