feat(nickel): emit_host scaffold — ProbedState → Nickel host config text

[UnbreakableMJ]

Summary

Opens M4 W1 reconcile direction (Plan §7.5) — the inverse of load_host. Where load_host shells out to nickel export and parses TOML into a DeclaredState, emit_host renders a ProbedState as Nickel host-config text intended to land at <config_dir>/hosts/<host>.imported.ncl as the starting point for an operator's reconcile review.

This first chunk emits only the always-present blocks: meta and kernel. Unprobed meta fields (timezone, arch_level, locale, keymap) get conservative placeholders that the operator hand-edits. Subsequent chunks (a) probe /etc/timezone, /etc/locale.conf, /proc/cpuinfo to populate them automatically and (b) add the packages, services, users, and config-file blocks.

The pure-function / no-I/O design keeps emit testable without the nickel binary; full round-trip via nickel export will be exercised once the engine method that wraps probe + emit ships.

• New pearlite-nickel::emit module + emit_host(probed: &ProbedState) -> String.
• Conservative defaults documented in rustdoc.
• Backslash and double-quote escaping for Nickel string literals.
• Five unit tests: basic emit, two escape paths, top-level brace shape, meta-before-kernel ordering.
• time crate added to dev-dependencies for fixture construction.

Test plan

• cargo test --workspace --all-features — 321 passing (+5 new)
• cargo clippy --workspace --all-targets --all-features -- -D warnings — clean
• cargo fmt --all -- --check — clean
• scripts/ci/check-spdx.sh — clean
• pearlite-audit check . — 1 check, 0 violations
• CI green on T1 / T2 / T3

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