fix(render): exit non-zero on error diagnostics from successful renders

[cscheid]

Braid strand: bd-zcjtaz78 (discovered during #362 / bd-yjs54ptg).

Problem

A render can complete successfully — output file written, no Pass-1/Pass-2 failure — while carrying a DiagnosticKind::Error diagnostic. Examples: a duplicate crossref identifier (Q-15-1, reported and skipped by the indexer), an include-file parse error (Q-5-3), or a Lua filter calling quarto.error(). Such a render printed Error: ... and tallied 1 error in the summary line, yet exited 0 — so CI reported success on output the user was explicitly told is broken.

Fix

should_exit_nonzero now reads ProjectRenderSummary::diagnostic_counts(), which aggregates every diagnostic source — pass-1/pass-2 failure diagnostics, project-level diagnostics, and per-document diagnostics on successful outputs (the previously-ignored source). Any error-severity diagnostic forces exit 1, with or without --strict. Warnings keep their existing semantics (exit 0 unless --strict); Info/Note never gate. The render still completes and writes its output; only the exit code changes.

The gate is now generic over OutputDiagnostics, which makes the per-output case directly unit-testable. Only crates/quarto is touched.

Behavior change for non-strict users, by design: CI pipelines that currently pass despite Error: output will start failing — the exit code now agrees with what the diagnostics already said.

Testing

• TDD: 3 CLI integration tests (crates/quarto/tests/integration/render_exit_codes.rs) spawning the real q2 binary against a duplicate-table-id fixture; confirmed failing first (observed 1 error with exit 0).

• 3 new unit tests for the gate's per-output arm (error / warning / info).

• cargo nextest run --workspace: 9895 passed — no existing test relied on the old exit-0 behavior. cargo xtask verify --skip-hub-build and clippy green.

• End-to-end, output inspected:

  $ q2 render duptbl.qmd     # two tables share {#tbl-dup}
  Error: [Q-15-1] Duplicate crossref identifier
  1 error
  # exit 1; duptbl.html still written
  

  Also verified a Lua filter's quarto.error() now yields exit 1, and a warnings-only document still exits 0.

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