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

The `Cargo.toml merge=union` .gitattributes (added while resolving the chained
example-PR conflict cascade) is the wrong tool for structured TOML. `union`
keeps both sides of a conflict hunk line-by-line, so 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 is worse than a normal conflict (which at least shows clear markers). CI
catches it, but only after a broken merge. For an append-only list of
multi-line blocks the right resolution is a plain 3-way merge (resolve markers
by keeping both blocks intact), not a line-union. Removing the driver.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MLBnPuScZy6w9di2QEjsXM

Author: claude