Drop a grounding read that a newer one replaced

[ColtenOuO]

Follow-up to #39.

Sumarry

loadGroundingFile wrote its result whenever the file read finished, even if a newer action had happened in the meantime:

• Second file on the same input: pick a slow JD, then a second JD before the first finishes. The slow read lands last and replaces the　second file's snippets, so the input shows one file and the list　another. If the slow read fails, it clears the good result instead.
• Clear during a read: press Clear document grounding while a resume is still reading. When the read lands, the cleared snippets and the "Parsed locally" status come back.

Fix and Change

Each document kind keeps a read counter (groundingReads.jd / .resume). A read takes the next number before its await and gives up afterwards if the number is no longer current, so it touches neither grounding, the status line, nor the lists. Clear bumps both counters. The counters are per kind so a JD read never cancels a resume read.

New test

Two lobbyTests in tests/browser/lobby.test.js. They hold the read of a file named slow.txt open by wrapping Blob.prototype.arrayBuffer, and release it on purpose.

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Summary by cubic

Fixes a race condition where a grounding file read that finished later could replace the result of a newer read, leaving the file list and snippets out of sync. Reads now use per-kind counters, and a read that is no longer the latest is dropped without touching the lists or status line.

Bug Fixes

• Picking a second JD before the first finishes reading no longer lets the slow read overwrite the second file's snippets.
• Clearing grounding while a resume is still reading no longer brings back the cleared snippets and "Parsed locally" status.
• Adds two lobby tests that hold a slow read open to cover both stale-read cases.

^{Written for commit 0f79ad5. Summary will update on new commits.}

[ColtenOuO]

The check failure here is not from this change. The only failing gate is cargo-audit, which flags rustls 0.23.43 under RUSTSEC-2026-0285 (TLS 1.3 handshake messages accepted across encryption level boundaries), published 2026-09-14. The advisory landed between runs: this branch passed in run 34843141378 and failed in run 34874694806 the same day.

The fix is rustls >= 0.23.45, in #42 . I will rebase this branch once that merges.

[jserv]

The fix is rustls >= 0.23.45, in #42 . I will rebase this branch once that merges.

Go ahead.

[ColtenOuO]

rebase to the main, CI looks green!