Raw storage tables: document the escape hatch and make it round-trip

Start with README raw-storage documentation, testdata/regression/raw-storage/main.md, and renderTable in internal/convert/storage_to_md.go. Read storage_to_md_test.go, especially TestRoundTripPassthrough and its raw-table case, then run the round-trip and forward-path regression tests. Done means documented raw-table behavior and stable read → publish → read output without losing unsupported table attributes.

Context

With cell background colors landing in #54, markdown covers the table features people ask for most. What it still can't express: explicit column widths (<colgroup>/<col>), data-table-width, a layout other than align-start (center/wide/full-width), rowspan/colspan, and per-cell alignment (that one's #48).

For those, the escape hatch is pasting a raw storage <table> into the markdown — the same thing we already tell people to do for macros and layouts. It works today, but it's undocumented, and read destroys it.

Part 1 — document it

Verified behavior on the publish path (probed locally, all three shapes come out intact):

• A <table> block passes through verbatim, including data-table-width, data-layout, <colgroup>/<col> widths, data-highlight-colour, rowspan, and <p style="text-align: right;">.
• Blank lines inside the table don't break it. The blank line is dropped and the structure survives, despite <table> being a goldmark HTML block (which normally ends at a blank line).
• Markdown inside a <td> is converted when separated by blank lines — the same convention as <ac:layout-cell>. Tight against the tags it stays literal.

Two gotchas worth documenting alongside, from the empirical table-format work:

• markfluence does not stamp its usual data-layout="align-start" onto a raw table. The author owns every attribute — including the layout.
• A <colgroup> on a table with no data-layout makes Confluence default the layout to full-width, so a raw table with column widths must always carry an explicit data-layout or it silently goes edge-to-edge. Also: px widths are honored with or without data-table-width, but percentage widths require it (they're resolved against it, and are silently ignored without it).

Deliverables: a README subsection under the raw-storage docs, and a regression case — testdata/regression/raw-storage/main.md currently contains no table at all, so nothing pins this behavior.

Part 2 — make it round-trip

renderTable in internal/convert/storage_to_md.go renders every table as a bare GFM pipe table and ignores all attributes. Reading a complex page proves the cost — on page 2913502220 (21 colored cells, three layouts, a distributed-column table), markfluence read returns:

| **header 1** | **header 2** |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- |
| data | data | data | data | data | data | data |

Everything is gone: all cell backgrounds, data-layout: center, data-table-width, the colgroup, and the text-align cell alignment. So read → edit → update silently strips a hand-built table — which makes the escape hatch a trap rather than a hatch, since it can't be documented as safe until this is fixed.

Proposed rule, consistent with how read already handles unmapped macros and column layouts:

1. If a table only uses what markdown can express, emit a GFM table. Cell backgrounds are now expressible, so map data-highlight-colour back to a <!-- bg:COLOR --> marker (preferring the swatch name, falling back to the hex). Alignment joins this list once #48 lands.
2. Otherwise emit the table verbatim as raw storage, so it round-trips exactly.

Deliverables: a raw-table case added to TestRoundTripPassthrough in storage_to_md_test.go (read → publish → read is stable), plus forward-path regression coverage.

Notes

• The alignment half of (1) depends on #48.
• Worth deciding whether the raw fallback should also fire for tables that are nearly expressible (e.g. a single rowspan) or whether to degrade those cells — the former is safer and simpler.