Rewatch: improve watch output and add clear-screen (#8373)

* Rewatch: show warnings in completion output

* Rewatch: use CompilationOutcome enum

* feat(rewatch): add watch --clear-screen

Make screen clearing opt-in so scripted stdout stays stable while
interactive watch mode can drop stale warnings and errors between
rebuilds.

Also print explicit rebuild and failure markers so fixed vs broken
state is easier to spot.

Refs #8139

Signed-off-by: Paul Kim <paul.bushuo@gmail.com>

* fix(rewatch): restore full rebuild plain output

Signed-off-by: Paul Kim <paul.bushuo@gmail.com>

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Signed-off-by: Paul Kim <paul.bushuo@gmail.com>