Close reader descriptor after open errors

[jeet-dekivadia]

Fixes #27.

Reader.open_ opened a descriptor and passed it directly into trace-header parsing. If a truncated or malformed trace raised during make_reader, the caller never received a reader value and had no descriptor to close.

Close the descriptor best-effort on the exception path and re-raise the original parsing exception. Successful readers retain ownership exactly as before.

The Linux regression opens an invalid trace ten times and checks /proc/self/fd before and after the loop, so the failure path must leave the descriptor count flat.

Validation:

• git diff --check
• private Linux runner: installed OCaml 5.2.1 and ppx_jane, then ran dune build @all -j2; compilation reaches the repository sources but the public toolchain cannot compile existing Domain.Safe and Sys.Safe uses and existing warning-as-error findings, so dune runtest is not reachable outside the Jane Street build environment

[jeet-dekivadia]

Small follow-up on this one: the patch is limited to the Reader.open_ exception path where the descriptor has already been opened but no reader is returned to the caller. The cleanup is best-effort and re-raises the original parse error, so successful reader ownership is unchanged.

The regression exercises repeated malformed opens and checks that /proc/self/fd stays flat. DCO is green. Happy to adjust the local shape if you would prefer this cleanup to sit somewhere else in the open path.

[jeet-dekivadia]

I rechecked this today while it is waiting for review. The diff is still narrowly on the Reader.open_ failure path after the file descriptor has been acquired and before successful reader ownership can be returned to the caller.

One detail I wanted to call out explicitly: the cleanup intentionally preserves the original parse/open exception and treats descriptor close as best-effort only on that failure path. The normal successful reader path is unchanged. DCO is still green.