fix: deleting a key added before a table leaves a blank line behind

[MildlyMeticulous]

Summary

Adding a key to a document that begins with a table inserts a blank line to separate the new key from [a]. Deleting that key leaves the blank line behind, so the round trip doesn't restore the original document.

doc = tomlkit.parse("[a]\nx = 1\n")
doc["foo"] = 10
del doc["foo"]
doc.as_string()   # 0.15.1 gives '\n[a]\nx = 1\n', expected '[a]\nx = 1\n'

The separator was stored as the table's indent, so it outlived the key it was inserted for. It now trails the inserted item and is removed along with it. Repeated inserts still yield exactly one blank line, the behaviour asked for in #178, and there's a test covering that as well.

1053 tests pass.

Agent Drafting Metadata

• Agent: Claude Code
• Model: claude-opus-5
• Notes: Found by round-trip differential testing against 0.15.1.