fix(codegen): per-category summary + hint when WIT requested but none emitted

[avrabe]

Summary

spar codegen previously printed a bare Generated N files in <dir>/,
which silently hid the most common confusing case: a model with
threads but no process subcomponents run under the default
--format both emits Rust + Cargo.toml + BUILD.bazel and zero
.wit files (because WIT generation is per-process). Operators and
agents read the output as "the other files but no wit" with no clue
why.

This is a no-functional-change diagnostic improvement: replace the
summary with a per-category breakdown and add a focused hint when WIT
was requested but none came out.

Before

Generating code for Threads::Sys. (2 components)
Generated 3 files in /tmp/threads-out/

After

Generating code for Threads::Sys. (2 components)
codegen: wrote 3 files (1 .rs, 2 workspace) (format: both) to /tmp/threads-out/
hint: --format both emits one .wit interface per AADL `process` subcomponent;
      this model has 0 processes — add `process` declarations to the system
      implementation, or pass --format rust for thread skeletons only.

The hint fires only when WIT was requested (--format wit or the
default --format both) and zero .wit files came out.

Smoke matrix

Case	Format	Expectation	Result
Threads-only model	both (default)	hint fires, (1 .rs, 2 workspace)	✅
Process + --format wit	wit	(1 .wit), no hint	✅
Threads + --format rust	rust	(1 .rs, 2 workspace), no hint (user didn't ask for WIT)	✅

Files

• crates/spar-cli/src/main.rs — cmd_codegen calls two new helpers:
  summarise_codegen_output and maybe_print_empty_wit_hint. Both
  are pure functions of &[GeneratedFile] + OutputFormat, suitable
  for golden-test promotion later if the stdout shape is contracted.
• Artifacts: REQ-CODEGEN-SUMMARY + TEST-CODEGEN-SUMMARY-HINT.

Test plan

• Three smoke cases above all match expectations
• cargo clippy -p spar --all-targets — clean
• cargo fmt --check — clean
• rivet validate — 0 broken cross-refs; totals byte-identical to baseline

🤖 Generated with Claude Code

[github-actions]

Rivet verification gate

✅ 20/20 passed

	count
Passed	20
Failed	0
Skipped (no steps)	0

Filter: (and (= type "feature") (or (has-tag "v093") (has-tag "v0100")))

Failed artifacts

(none)

_{Updated automatically by tools/post_verification_comment.py. Source of truth: artifacts/verification.yaml.}