fix(lading): surface tooling/FUSE-mount panics as errors

[goxberry]

Summary

Third and final PR in the FIXME-conversion stack. Addresses the three
remaining sites: two in the offline captool analyzer, one in the
FUSE-mount setup of the logrotate-fs generator.

bin/captool/main.rs — main and validate_capture

Both functions consume a LinesStream via .map(|l| { ... }) and
panic via .expect() on either a failed line read or a malformed
JSON record. Convert the closures to return Result<Line, Error>
and fail-fast on the first error. captool is an offline analyzer —
silently skipping corrupt lines would mask the corruption being
investigated.

The fn-level #[expect(clippy::expect_used)] stays on both
functions: a few .expect() calls remain on tokio::task::JoinHandle
results, where a JoinError is the intentional propagation of an
inner blocking-task panic. The reason text is rewritten so the
attribute no longer reads as tracked debt — it now describes
deliberate panic propagation.

generator/file_gen/logrotate_fs.rs::Server::new

fuser::spawn_mount2 returns an io::Result and the existing
Error::Io variant is already #[from] std::io::Error. The fix is
trivial: replace .expect("Failed to mount FUSE filesystem") with
?. No new variant needed. The fn-level FIXME #[expect] is
removed.

End state

After this PR, grep -rn 'FIXME' lading/src/ returns no results. All
remaining #[expect(clippy::expect_used)] attributes in the lading
crate are correctly-documented intentional panics (channel-iterator
construction, OnceCell set-once, JoinHandle task-panic
propagation, tokio::select! branch invariants, etc.) rather than
tracked debt.

Test plan

• ./ci/validate passes
• grep -rn 'FIXME' lading/src/ is empty
• cargo build --workspace --all-targets --all-features

🤖 Generated with Claude Code

[goxberry]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(lading): surface tooling/FUSE-mount panics as errors #1904 👈 (View in Graphite)
• fix(lading): surface remote-input parsing failures as errors #1903
• fix(lading): surface user-config parsing failures as Error variants #1902
• refactor(lading): drop expect_used quarantines and annotate bins #1901
• refactor(lading): annotate intentional-panic observer/target sites #1900
• refactor(lading): annotate intentional-panic network generators #1899
• refactor(lading): annotate intentional-panic file_gen sites #1898
• refactor(lading): annotate intentional-panic blackhole + small generator sites #1897
• refactor(lading): convert structural-infallible .expect() to unreachable!() #1896
• refactor(capture): retire .expect() sites and drop quarantines #1894
• refactor(payload): retire remaining .expect() sites and drop quarantine #1893
• fix(payload): reject zero-byte arbitrary input for Block #1892
• refactor(payload): annotate intentional-panic .expect() sites #1891
• refactor(payload): mark in-function invariants as unreachable!() #1890
• refactor(payload): mark fallible-context infallibles as unreachable!() #1885
• refactor(payload): mark infallible .expect() sites as unreachable!() #1884
• chore(clippy): drop expect_used quarantine for lading_throttle #1883
• chore(clippy): forbid .expect() in production code #1882
• main

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