Render a declared limit through the manifest read path (#78)

[sehkone]

Summary

The bump to manifest format version 5 was cut so a producer's manifest could carry limit_nofile, but nothing asserted that end. The wire form was exercised for the two refusals and for the floor version that emits no directive; the positive case reached the renderer only from a hand-built template, so the read path this bump exists for was never driven end to end.

This adds a_manifest_at_the_current_version_renders_a_declared_descriptor_limit in src/manifest.rs: a manifest at MANIFEST_FORMAT_VERSION declaring limit_nofile parses, the decoded spec carries the value, and the renderer emits LimitNOFILE=8000 on the line after RestartSec=, asserted against the full unit text. It mirrors the test the previous format bump left behind for the manager endpoint.

No production code changes.

Part of #78

Not addressed

Everything else the issue asked for — the limit_nofile field on UnitTemplate, the two ModuleSpecError variants rejecting Some(0) and Some(u64::MAX), the LimitNOFILE= emission between RestartSec= and the sandbox booleans, the producer/ceiling version bump with the floor held still, the rewritten check_format_version doc comment, and the rest of the test plan — already landed on main in #80, which closed the issue. This branch carries only the one read-path assertion that work left uncovered, so nothing in the issue is outstanding once it merges.

Test plan

• cargo fmt -- --check --config group_imports=StdExternalCrate is clean
• cargo clippy --all-targets -- -D warnings and cargo clippy --all-targets --features test-support -- -D warnings are clean
• cargo test and cargo test --features test-support pass
• A manifest at MANIFEST_FORMAT_VERSION declaring limit_nofile parses, and the decoded UnitTemplate carries the value
• That decoded spec renders LimitNOFILE=8000 in [Service] on the line after RestartSec= and before the [Install] section
• The existing floor-version test still asserts a manifest at MIN_MANIFEST_FORMAT_VERSION renders no Limit line
• The byte-for-byte golden tests for templates that set no limit pass unchanged

[sehkone]

[Reviewer Round 1]

Approved — no findings.

The new test in src/manifest.rs genuinely exercises the missing producer-wire-form path: it parses a v5 manifest containing limit_nofile, confirms the decoded field, then asserts the complete rendered unit including LimitNOFILE=8000.

It complements the existing renderer placement and floor-compatibility tests without broadening scope. PR #82’s Part of #78, ## Not addressed, and test plan accurately reflect that #80 already closed the implementation issue.

[sehkone]

[Review Verdict Round 1: APPROVED]