feat(convert): read a table Markdown cannot express as raw storage

[willkg]

Closes #55.

What changes

read and export used to write every table as a GFM pipe table and drop whatever did not fit: a layout, column widths, merged cells, a headerless shape, block content in a cell. The next publish then stripped those from the page. Now a table reads back as a pipe table only when Markdown expresses all of it, and otherwise as raw storage with each cell's body as Markdown between blank lines, the convention layout cells already use:

<table data-layout="center">
<tbody>
<tr>
<td rowspan="2">

**auth** is [up](https://status.example.com)

</td>
...

The decision is an allowlist (internal/convert/storage_to_md_table.go), so markup nobody has seen yet keeps a table raw rather than being dropped. Publishing already passed a raw table through intact, so nothing changes on the write side. The plan is _plans/055_raw-tables-round-trip.md, with every amendment made during implementation at its end.

markfluence's own tables stay Markdown

Tested with a real browser save: a table create published, then edited in the Confluence editor, reads back as exactly the Markdown it was created from. The editor writes ids, a bare local-id on every paragraph, data-table-width, and on an align-start table a <colgroup> of widths it measured; read ignores all of these. The cost: a column someone resizes by hand writes the same <colgroup>, so the resize is lost on the next publish. A <colgroup> on any other layout, and any layout other than align-start, keeps a table raw, since those are visible choices markfluence never makes.

Tables markfluence did not write mostly come back raw. In a sample of 152 tables edited since June, most use the default or full-width layout. That is the trade: keeping what is on the page over pretty Markdown.

Measured, not assumed

Everything about what takes effect was probed against the live instance with ADF, and recorded in docs/confluence/storage-format.md: which table markup does anything (a <caption> leaks its text above the table; a nested table becomes something the editor cannot edit), what the editor writes on a table, and how a paragraph's alignment combines with its cell's. Only center and right do anything; left and justify do nothing, and start and end are stripped, so a paragraph saying any of those four takes its cell's alignment. A cell's shared alignment is written on the cell (<td style="text-align: center;">) so its paragraphs stay Markdown.

Property test

internal/convert/table_property_test.go generates tables from a seed and checks that each reads, gives Markdown that is a fixed point after one publish, and publishes back to the same model of what takes effect in Confluence. 3000 seeds run in every make test (about 1.5 s); FuzzTableProperty runs more on demand, and about 610,000 passed; TestTablePropertyCorpus runs the same checks over real tables from a local file. Over the 148 tables in a personal space, the only two failures were outside the table rules, and are #203 and #204. The model has its own readings of every rule it checks, so a mistake in a converter helper fails the test rather than being shared by it.

Changes outside tables

These came out of the reviews and the property test, and touch more than tables:

• The bare local-id the editor writes is now dropped everywhere, beside ac:local-id.
• renderRawBlock writes an element holding loose text whole, where it used to drop the text (layouts and unknown macros too).
• In a pipe cell, renderCellLines no longer puts a <br> beside a list (it published as a blank line), and a list containing a | now keeps a table raw (it split the row and broke the whole table).
• docs/markdown-file.md wrongly said a multi-line cell publishes back to paragraphs; it publishes line breaks, which look the same.

Found and filed, not fixed here

• read: paragraph text that looks like a Markdown block becomes structure on publish #203: paragraph text that looks like a Markdown block (1. , # , > ) becomes that block on publish.
• read: a trailing space inside bold, italic or strikethrough is dropped, joining words #204: a trailing space inside bold, italic or strikethrough is dropped, joining words.
• read: two lists side by side come back as one list #205: two lists side by side come back as one.

Not in scope

A warning when a table falls back to raw; a compact one-line form for a plain-text raw cell; an empty-header convention for headerless tables; a check lint for a <colgroup> with no layout, a <caption> or a nested table. Each waits for someone to need it.