Coalesce repeated per-page render diagnostics + restore config source snippets (bd-mg3ckvp7)

[cscheid]

Problem

A problem anchored in a shared file is re-diagnosed by every page's render pipeline and printed once per page. On the connect-docs testbed (external-sources/connect-docs/docs-quarto-2): 186 identical Q-13-2 "Navbar references missing document" warnings, one per rendered page, all caused by a single bad navbar href in _quarto.yml — and none of them naming the source file, because the per-document SourceContexts never register _quarto.yml's hash-based FileId.

Changes

Building on bd-9hlja (which coalesced pass2_failures only and explicitly deferred this case):

1. RenderToFileResult.input_path (quarto-core): successful-render results now carry their input path — the exact gap bd-9hlja recorded as the blocker.
2. Coalesced emission (crates/quarto/src/commands/render.rs): per-page diagnostics from successful renders route through coalesce_by_source from quarto-error-reporting 0.2.1, whose path-aware LocationKey (posit-dev/quarto-error-reporting#3) makes cross-document grouping safe for sequential per-doc FileIds. One emission per distinct source span, with an Affected files: tail. Print-only: diagnostic_counts(), --strict promotion, exit codes, and --json-errors are unchanged.
3. Config snippet restoration (attach_config_source): when a coalesced group's location FileId matches quarto_yaml::file_id_for_filename(config_path) and isn't resolvable in the carried context, the config file's content is registered under that id so the ariadne snippet renders. Best-effort; any failure degrades to the previous span-less block.

Dep bump: quarto-error-reporting 0.1.0 → 0.2.1 in the workspace and in crates/wasm-quarto-hub-client/Cargo.toml (independent pin outside the workspace).

Result on the testbed

Q-13-2 went 186 → 1, and the single warning now points at the offending line:

Warning: [Q-13-2] Navbar references missing document
    ╭─[ …/docs-quarto-2/_quarto.yml:44:15 ]
 44 │         file: api/index.qmd
    │               ──────┬──────
    │                     ╰──────── 'api/index.qmd' is not in the project index.
────╯
ℹ Check the spelling, or confirm the target file is included in the render set.
Affected files: …/admin/access-controls/index.qmd, … (and 183 others)

Exit code unchanged (pre-existing Q-5-3 errors). Remaining repeated classes (Q-12-7 ×15, Q-2-9 ×88, unknown shortcodes ×75) verified to be genuinely distinct source locations.

Tests

4 new integration tests in crates/quarto/tests/integration/coalesced_diagnostics.rs, all written red-first:

• coalescing across pages (exactly one Q-13-2, tail names all pages)
• (and N others) display cap
• singleton keeps the legacy no-tail shape
• config-anchored group renders the _quarto.yml snippet

Verification: full workspace suite (10,810 passed), full cargo xtask verify exit 0 (covers the 0.2.1 bump through the WASM/hub legs), final cargo xtask verify --skip-hub-build exit 0, clippy + fmt clean.

Plan + full diagnosis: claude-notes/plans/2026-07-31-repeated-diagnostics-coalescing.md. Known pre-existing flake collect_reverification_skips_rereferenced_candidate (bd-ce1mv6xv) surfaced once during local verification; unrelated to this diff.

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