Revert Cargo.toml union-merge driver — it mangles [[example]] blocks

[AdaWorldAPI]

What / why

While resolving the chained example-PR conflict cascade, I added a .gitattributes with Cargo.toml merge=union. That was a mistake — union is the wrong tool for structured TOML and this PR removes it.

union keeps both sides of a conflict hunk line-by-line. When two branches add an [[example]] block at the same anchor, it interleaves them into a single block with two name keys:

[[example]]
name = "gridlake_field_tile"
name = "subpel_tap_tile"        # ← duplicate key → cargo refuses to parse
required-features = ["std"]

That's worse than a normal conflict — a plain 3-way merge shows clear markers you resolve by keeping both blocks intact, whereas union silently produces broken TOML (caught by CI only after the fact). It reproduced exactly this way during the last rebase of #235.

For an append-only list of multi-line blocks, plain 3-way merge is correct. Removing the driver.

Docs/tooling only — no code change.

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