refactor(lading): drop expect_used quarantines and annotate bins

[goxberry]

Summary

Final sub-PR of the lading-crate cleanup. Annotates the three bin entry points (lading, captool, payloadtool), drops the four #![allow(clippy::expect_used)] quarantines, and annotates the previously-masked production sites in generator/{process_tree,tcp_rr,udp} that the quarantine drop surfaces.

After this PR, the lading crate has no remaining crate-level expect_used quarantines — every .expect() is either covered by a fn-level #[expect] with a documented reason or has been mechanically converted to .unwrap_or_else(|| unreachable!(...)) in an earlier sub-PR of this stack.

Quarantines dropped:

• `lading/src/lib.rs`
• `lading/src/bin/lading.rs`
• `lading/src/bin/captool/main.rs`
• `lading/src/bin/payloadtool.rs`

New fn-level annotations:

• `bin/lading.rs::{CliKeyValues::from_str, get_config, inner_main}`
• `bin/captool/main.rs::{main, validate_capture, report_validation_result}`
• `bin/payloadtool.rs::{generate_and_check, check_generator}`
• `generator/process_tree.rs::spawn_tree`
• `generator/tcp_rr.rs::{spin, client_thread_main}` (FIXME on spin — user-supplied addr)
• `generator/udp.rs::{Udp::new, UdpWorker::spin}` (FIXME on Udp::new — user-supplied addr)

Stacked on #1900. With this stack landed, the lading crate is ready for clippy::expect_used = "deny" enforcement to be flipped on workspace-wide.

Test plan

• `./ci/validate` passes
• CI green on this PR

[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
• 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 👈 (View in Graphite)
• 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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