refactor(payload): mark fallible-context infallibles as unreachable!()

[goxberry]

What does this PR do?

Converts 5 .expect() sites in lading_payload to .unwrap_or_else(... unreachable!("...")). The sites all live inside Result-returning functions but are themselves guarded by local invariants — propagating with ? would introduce dead error paths.

Sites

File:line	Site	Local invariant
common/strings/string_list_pool.rs:179	start.parse::<u32>().expect(...)	start_is_num was set to start.parse::<u32>().is_ok() 27 lines above
common/strings/string_list_pool.rs:182	end.parse::<u32>().expect(...)	same, for end
common/strings/string_list_pool.rs:192	start.chars().next().expect(...)	start_is_char = start.len() == 1 && !start_is_num
common/strings/string_list_pool.rs:196	end.chars().next().expect(...)	same, for end
templated_json/resolver.rs:155	self.raw_defs[idx].take().expect(...)	three-color visit machine: reachable only when !black[idx] && !grey[idx], which corresponds to raw_defs[idx] being Some

Why unreachable!() and not ?

An earlier draft of this PR used ?-propagation (the agent's initial recommendation, since the containing functions return Result). On review the choice changes: these aren't real error paths — the parses cannot fail because start_is_num was set from a successful parse::<u32>().is_ok(), and chars().next() cannot return None on a string the local code has just established has len() == 1. unreachable!() documents that fact; ? would leave a dead Err(Error::MalformedPattern{...}) branch behind that's never produced.

These sites fit the same shape as the previous unwrap_or_else(|| unreachable!()) refactor (#1884), but were initially split out because they live in Result-returning functions. They're being landed in their own PR for review clarity, not because they need a different treatment.

Motivation

Third per-crate cleanup-PR in the stack started by #1882. Future PRs in this sub-stack:

• Group B (next): 6 sites in block.rs, random_string_pool.rs, string_list_pool.rs where the invariant is established in the same function (same pattern as this PR; separated by the "originally in a Result fn" distinction).
• Group C: ~16 sites where the .expect() is the contract (handle-table lookups, documented API panics) — annotated with #[expect(clippy::expect_used, reason = "...")].
• Group D: real bug fix in block.rs:123 (the arbitrary::Arbitrary impl for Block can panic on u32::arbitrary returning 0).
• Final: drop the lading_payload quarantine.

Related issues

Stacked on #1884.

Additional Notes

• cargo build --all-targets --all-features ✓
• cargo clippy --all-targets --all-features ✓
• cargo test -p lading-payload --lib ✓ (244 passed)
• No public-API change. Diff: 3 files, +12 −5.

[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
• 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 👈 (View in Graphite)
• 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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