Fix qmd reader doubling backslashes in attribute values (issue #161, bd-tpjg)

[cscheid]

Fixes #161.

Summary

The qmd writer emits Pandoc-style escapes (\ → \\, " → \") inside "..." attribute values, but the qmd reader (extract_quoted_text in crates/pampa/src/pandoc/treesitter_utils/text_helpers.rs) only un-escaped \" / \' and left every other \X verbatim. So each qmd → JSON → qmd cycle doubled the backslashes (\[ → \\[ → \\\\[ → ...), and any document with tbl-colwidths="\[N,N\]" accumulated backslashes on every round trip.

The asymmetry was on the reader side. The writer already matched Pandoc.

Change

extract_quoted_text now applies Pandoc's CommonMark rule: \X collapses to X when X is ASCII punctuation, otherwise the backslash is preserved literally. A trailing \ is also preserved. The same helper is used for CommonMark link titles, which follow the same escape convention, so the change is consistent for both call sites.

This changes the AST for inputs that contained backslashes inside attribute values — any consumer that relied on backslashes being preserved verbatim will see them collapsed (matching what Pandoc has always produced). Detailed before/after analysis in claude-notes/issue-reports/161/triage.md.

End-to-end verification

$ printf -- '::: {data-foo="\\[1,2\\]"}\nhello\n:::\n' | cargo run --quiet --bin pampa
[ Div ( "" , [] , [("data-foo", "[1,2]")] ) [Para [Str "hello"]] ]

$ printf -- '::: {data-foo="\\[1,2\\]"}\nhello\n:::\n' | cargo run --quiet --bin pampa -- -t qmd
::: {data-foo="[1,2]"}

hello

:::

Round-trip is now stable (matches Pandoc's markdown -> markdown output for the same input).

The original tbl-colwidths="\[40,60\]" pattern from the linked quarto-web docs was also exercised end-to-end and now stabilizes to tbl-colwidths="[40,60]" as expected.

Tests

• Three new round-trip fixtures under crates/pampa/tests/roundtrip_tests/qmd-json-qmd/:
  • attr_value_backslash_escaped_punct.qmd — the reported case (\[1,2\])
  • attr_value_literal_backslash.qmd — literal backslash via \\
  • attr_value_escaped_quote.qmd — already-working \" case, locked in as a regression test
• Unit tests in text_helpers.rs for extract_quoted_text covering quoted/unquoted, punctuation/non-punctuation, and trailing-backslash edge cases.

cargo xtask verify --skip-hub-build --skip-hub-tests --skip-trace-viewer-build --skip-trace-viewer-tests is clean (8417 Rust tests passing). The skipped JS steps are unrelated fresh-worktree bootstrap (no Rust→JS code path is touched here).

Test plan

• Spot-check that the AST change doesn't break any downstream consumer that was working around the previous behavior (none expected, but worth a glance at quarto-web rendering).
• If a reviewer wants stricter assurance, run cargo nextest run --workspace locally — already green here.

Commits

• d31ae2dd — Triage record (claude-notes/issue-reports/161/).
• 7b0f7e91 — Fix + tests.

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