refactor(lading): annotate intentional-panic observer/target sites

[goxberry]

Summary

Annotate intentional-panic .expect() sites across the observer subsystem, target launcher, target metrics scrapers, inspector, neper threading, and the captool analyzer. Reasons document the invariant (telemetry channel, cgroup ABI, paired-map structure, etc.); FIXME markers track sites that should eventually surface as Error variants.

Sites annotated:

• `observer.rs::run` — telemetry channel invariant
• `observer/linux/procfs.rs::{handle_process, proc_exe}` — process_info map invariant; UTF-8 exe path
• `observer/linux/cgroup/v2/cpu.rs::poll` — cgroup v2 kernel ABI
• `observer/linux/procfs/memory/smaps.rs::into_region_strs` — const regex
• `target.rs::{watch_container, execute_binary}` — PID i32 conversion
• `target_metrics/expvar.rs::run` — reqwest TLS bootstrap
• `target_metrics/prometheus.rs::{LABEL_REGEX, parse_prometheus_metrics}` — const regex + FIXME for parser panic
• `inspector.rs::run` — PID i32 conversion
• `neper/thread.rs::spawn_named` — OS thread alloc
• `bin/captool/analyze/jsonl.rs::analyze_metric` — paired-map invariant

Stacked on #1899. Part of the broader push toward enforcing clippy::expect_used = "deny" 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
• refactor(lading): annotate intentional-panic observer/target sites #1900 👈 (View in Graphite)
• 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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