fix(lading): surface user-config parsing failures as Error variants

[goxberry]

Summary

First PR in a three-part follow-up to the lading-crate expect_used
cleanup stack (#1896–#1901). Each of the 10 sites annotated with
#[expect(clippy::expect_used, reason = "FIXME: …")] in that stack
gets converted into a proper Error variant in one of three PRs,
grouped by failure mode.

This PR addresses the 5 user-supplied config sites — fields read out
of YAML at startup, where a malformed value should surface as a typed
error from the generator constructor rather than abort the process.

File	Site	Conversion
generator/tcp.rs	Tcp::new addr lookup	Add AddrParse { addr, source } + EmptyAddrResolution(String); ?
generator/udp.rs	Udp::new addr lookup	Same two variants on udp::Error; ?
generator/tcp_rr.rs	TcpRr::spin addr	Add InvalidAddr(#[from] AddrParseError); ?
generator/grpc.rs	Grpc::new URI + path	Add InvalidTargetUri { uri, reason } covering both Uri parse and missing path; ?
bin/payloadtool.rs	check_generator zero cache	anyhow::bail! (bin uses anyhow)

The fn-level #[expect(clippy::expect_used)] attribute is removed
from each of the five functions; no other .expect() calls remain in
their bodies.

Test plan

• ./ci/validate passes
• grep -n 'FIXME' lading/src/generator/{tcp,udp,tcp_rr,grpc}.rs lading/src/bin/payloadtool.rs 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
• fix(lading): surface remote-input parsing failures as errors #1903
• fix(lading): surface user-config parsing failures as Error variants #1902 👈 (View in Graphite)
• 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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