docs: gate rustdoc, and document the public API it was hiding

[vyncint]

Milestone 2.2.0, issue #57 — the last one.

The count was wrong by 5.6×

The issue said "about 42 undocumented public items". It is 237:

crate	items
launchbound-bench	74
launchbound-report	50
launchbound-prune	40
launchbound-space	29
launchbound-build	22
launchbound-model	15
launchbound-metal	4
launchbound-search	3

The reason I undercounted is instructive: I counted items and missed fields. 144 of the 237 are public struct fields — and those are the ones that matter most, because they are the wire formats. Every field of Report, Results, BenchPlan, Summary and FindingsDoc is something a consumer reads out of JSON and has to interpret. Documenting Summary::median_ms is not ceremony; it is the difference between a reader knowing that ranking uses the median rather than the mean, and guessing.

By kind: 144 struct fields, 29 structs, 25 variants, 15 methods, 10 associated functions, 9 enums, 4 functions, 1 type alias.

The gate found bugs in the commit that added it

cargo doc --workspace --no-deps was already at zero warnings, so the gate cost nothing to turn on — #![warn(missing_docs)] in the eight library crates plus a docs recipe in just ci.

It failed immediately on two broken intra-doc links I had just written: one to read_findings, a function whose real name is read_stream, and one to a private enum variant. A doc comment naming a function that does not exist is worse than no comment, and nothing before this would have said so.

Two things found while writing it

• launchbound-prune exports FindingsDoc but not read_stream or ReadError. The entry point for reading analyzer output is unreachable from outside the crate, so a consumer holding a FindingsDoc cannot use the function that produces one from a real run. I documented it as internal rather than quietly exporting it — adding public API is a deliberate act now that ci: gate the public API against the published release, and stop the pins drifting #63's semver gate exists, and it does not belong in a docs commit. Worth its own issue if you want it exported.
• device() gains a runnable example, per the issue's ask for one on the public entry points.

Verified

Appending an undocumented pub fn produces error: missing documentation for a function and fails the build. just ci green (now with docs between test and deny).

Closes #57

Signed-off-by: Vyncint Ng 115854244+vyncint@users.noreply.github.com