fix(docs): markdown 3+ tables drift placeholder offset by one per table

[chrischall]

Closes #607.

Root cause

The per-table offset accumulator in both replaceDocsMarkdownRange and insertDocsMarkdownAt:

if tableEnd > tableIndex {
    tableOffset += (tableEnd - tableIndex) - 1
}

assumed the placeholder "\n" we wrote into plainText for each table position was consumed by the InsertTable call. Empirically that placeholder remains in the doc — InsertTable inserts the new table BEFORE the existing character at the requested index, so every subsequent placeholder shifts forward by the full (tableEnd - tableIndex), not (tableEnd - tableIndex) - 1.

That -1 accumulated one missing character of drift per table:

Table	Cumulative drift
1	0
2	1
3	2

Why the issue only became visually obvious at the 3rd table: with 1 char of drift, table 2 lands on the blank line between paragraph and table, which is itself a newline — the doc still parses, you just lose the visual gap. With 2 chars of drift, table 3 lands inside the preceding paragraph's terminator/punctuation, splitting three.\n so the period becomes a standalone paragraph after the table.

Fix

Extracted the formula into a small helper:

func nextTableInsertOffset(currentOffset, tableIndex, tableEnd int64) int64 {
    if tableEnd <= tableIndex {
        return currentOffset
    }
    return currentOffset + (tableEnd - tableIndex)
}

and replaced both inline call sites with tableOffset = nextTableInsertOffset(tableOffset, tableIndex, tableEnd). The helper lives next to the rest of the table-insertion machinery in docs_table_inserter.go.

Tests

• TestNextTableInsertOffset_AccumulatesFullTableSize — exercises a 3-table sequence with hand-computed expected offsets and explicitly calls out the previous buggy values in the failure messages.
• TestNextTableInsertOffset_ZeroSizedTableLeavesOffsetUnchanged — guards the tableEnd <= tableIndex short-circuit (previously hidden inside an if).
• TestNextTableInsertOffset_MatchesIssueRepro — parameterised over any table size G, asserts N tables of size G accumulate to NG (previous formula gave N(G-1)).

go test ./internal/cmd/ -count=1 → ok 23.8s.