Bumps the version to 0.8.0 and assembles the four changelog.d fragments
into a dated 0.8.0 section, deleting them in the same commit.

Minor rather than patch: the fragments carry `### Added` bullets that
name new public surface. `Migrations.down/1` takes a `from:` option, V04
joins the Ecto DDL and moves `@current_version` to 4, and a chart can now
tag its own run failed through a top-level final's `<donedata>`. Nothing
is removed and no existing signature changes, so no major.

The README install pin moves from `~> 0.7` to `~> 0.8`: it carries the
major/minor form with the patch component dropped, and a minor moves it.

The lead paragraph is the only prose written at release time and names
the four user-visible changes; every bullet is carried over byte for
byte, grouped Added, Changed, Fixed. The section date is the local date
of this machine (`date +%F`), per the fleet's 2026-09-06 convention.

Also folds sp-23z's first item, which the README edit reaches by right:
line 437 framed `sp-461` as a number for how long the plain index build
takes. It is not - it measures the settlement read cost against a
GIN-indexed `metadata` column at increasing fan-out widths, which is what
ADR-0006's V04 Note says. The sentence now says so. sp-23z's remaining
items are in `docs/adr/`, which this commit does not touch, and sp-4ec is
in `CLAUDE.md`, which a prep never touches - both stay open.

CLAUDE.md's released-version line is left alone: it states what is
published, which stays true until the operator publishes.

No tag and no publish: those stay with the operator.

Refs: sp-5qd