Scrub all S-suffix source-info keys in round-trip JSON comparison (bd-j9wp)

[cscheid]

Summary

remove_location_fields in crates/pampa/tests/test.rs (used by test_qmd_roundtrip_consistency to compare JSON1 vs JSON3 modulo source info) was scrubbing only attrS, targetS, citationIdS — but the JSON writer emits seven more S-suffix keys for table internals: bodiesS, bodyS, captionS, cellsS, footS, headS, rowsS. captionS in particular is a scalar foreign-key into astContext.sourceInfoPool that sits directly on the Table object, so it survived the existing scrub as a dangling integer reference and could fail an otherwise content-stable round trip.

This was discovered while implementing #165 (issue #162); the table fixture there had to be dropped because of this gap and is reinstated in this PR.

Not a determinism issue

Verified across 5 repeated runs that the same input always produces the same captionS:N on JSON1 and the same captionS:M on JSON3. The failure was deterministic but unscrubbed, not flaky:

• JSON1's astContext.sourceInfoPool has 23 entries derived from the original qmd's source positions.
• JSON3's pool has 21 entries derived from the regenerated qmd's source positions.
• Both pools are deterministic; the writer regenerates qmd in a stable way, but the layout differs from the original (different cell padding etc.), so the pool contents and traversal-order IDs differ.
• After scrubbing astContext (which removes the pools themselves), the bare S-IDs are references to nothing, but the test compared them numerically anyway.

Change

• Extend the scrub list in remove_location_fields to cover all ten S-suffix keys the JSON writer emits today (grep -oE '"[a-zA-Z]+S"' crates/pampa/src/writers/json.rs is the source of truth).
• Add a doc-comment on remove_location_fields calling out the S-suffix convention so future writer-side additions extend this list too.
• Restore crates/pampa/tests/roundtrip_tests/qmd-json-qmd/table_with_inline_nbsp_in_cell.qmd as a regression fixture. It failed at HEAD on the captionS:17 ≠ captionS:15 mismatch and passes after the scrub-list extension.

Verification

• TDD: fixture failed at HEAD with the expected captionS mismatch, passes after the fix.
• cargo nextest run -p pampa: 3685 tests passing.
• cargo xtask verify --skip-hub-build --skip-hub-tests --skip-trace-viewer-build --skip-trace-viewer-tests: clean.

Risk

Adding more scrubs is monotone-safe: a previously-passing fixture cannot start failing because the comparison only became less strict. The theoretical risk is that a future writer-side change names a content-bearing key with S-suffix, and this list silently swallows it. The doc-comment on remove_location_fields flags the convention so a future reviewer extending json.rs sees the dependency.

Test plan

• Confirm there is no expectation elsewhere in the repo that any S-suffix key carries content rather than source info.

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