Split test-examples into two phases: build all examples in parallel
first, then run tests serially against pre-built binaries. Abort on any
build failure instead of skipping tests for that example.

Replace the centralized `examples/test-examples.toml` with individual
`test.toml` files in each example directory, updating the test runner
and related documentation accordingly.

Add breaking change for `with_dispatcher` removal, renumber feature
items, and outline automated `dispatcher_tree` optimization plan.

Replace the separate code/docs CI jobs and local check flags with
a unified `--check-*` flag system, enabling each check to run
independently across a platform matrix.

Update changelog and issue tracker notes for the enhanced
`default_completion` behavior.