Bumps `@version` in `mix.exs` and the compiler's second carrier,
`@compiler_version` in `lib/statifier_blocks/compiler.ex`, to 0.26.0,
and the README install pin to `~> 0.26`. The compiler version moves on
every release because it is stamped into every compilation record; no
compiled chart bytes move in this one.

Promotes the seventeen `changelog.d/` fragments into a `## [0.26.0]`
section dated with the local date, grouped by heading and in
fragment-name order within each, and deletes them: sb-1eam, sb-2qm3,
sb-4sek, sb-5i4p, sb-6xkf, sb-8fa8, sb-9w7w, sb-a0xw, sb-ako9, sb-ba15,
sb-cvtx, sb-h0nt, sb-mulk, sb-q183, sb-spn7, sb-u12s, sb-uzly. Every
bullet is carried over byte for byte; the lead paragraph and the link
reference are the only prose written here.

A minor, about what a host does around a composite: `Composite.Collapse`
reading a selected arrangement back as the declaration that stands for
it and the editor's "Save as a step" handing it to `on_collapse`,
pass-through slots on module and data composites, a declarative
`"migrations"` chain on a data declaration, all nine field types
spellable as JSON, `Composite.io/2` and `outcomes/2` through a palette,
and the `ViewModel`, `Edit.Targets` and `Environment` promotions the
host seams asked for.

Fills the README for what a host needs: the "Save as a step" gesture
and its `on_collapse` seam row, the pass-through slots bullet that
replaces the "a composite exposes no slot of its own" claim they
falsify, the data declaration's `"slots"` and `"migrations"` keys, and
the two assigns the mount gained - `on_collapse` and `debounce`, both
re-read against `editor.ex`'s own table. Drops the two "Not yet"
bullets this release falsifies.

Refs: sb-4zn8