docs: consolidate RELEASE.md around the unreleased Expr compiler

The 0.6.1 → HEAD log has 100+ commits, almost all incremental milestones
of the same feature — the native Expr compiler. The next release is a
single coherent story, not a sequence of nine bullets, so collapse Added
to: one bullet for the compiler itself (single-NIF replay, full
coverage, fallback semantics, end-to-end model results), one for the
`:fuse` opt-in, one for `Emily.Generation`, plus the orthogonal
async_eval and fast_rope_int additions. Drop the per-PR enumeration of
op clusters — they belong in commit messages, not in changelog.

Move the tuple-returning `cond` crash out of Fixed: it was a bug
introduced and fixed during the Expr compiler's own development, so from
a released-user perspective it's unreleased work absorbed into the
compiler bullet. The Fixed section keeps the real regression in the
released eager backend — dilated window reductions returning wrong
values, which the new compiler shares via the same window-reduce core.

Author: ausimian