Bumps @version to 0.12.0 and moves the README install pin to `~> 0.12`.
The minor, not a major: this is a breaking release - the `run` noun is
retired across the public surface, the durable table and its identity
column - but the package is pre-1.0, where the operator's standing
position is that a breaking change rides a minor rather than forcing a
1.0 the API is not ready for. The fragments' headings are Changed and
Deprecated with no Added, which is the evidence for a rename release
rather than a feature one.

Promotes changelog.d/sp-op4.md (Changed, Deprecated), sp-j2y.md
(Changed) and sp-tae.md (Changed) into a `## [0.12.0] 2026-09-13`
section and deletes all three. Bullets are carried over byte for byte,
grouped Changed then Deprecated, sp-op4's first - so the Changed group
opens with the bold breaking line and the adapter rename table. The lead
paragraph is the only prose written here: it names the release kind and
calls the breaking change out in bold, as every released section in this
file does and as 0.4.0 does for the bold callout. The heading carries no
`-` separator, matching every other section, and the date is this
machine's local date. No link-reference block is added; this CHANGELOG
has none.

Gate: full `mix quality` green on this exact tree (format, compile with
warnings-as-errors, deps, credo, 552 tests at 95.5% coverage, dialyzer);
Postgres answered from a local server on 5432, not docker compose.

Acceptance re-checked at this commit: `grep -n "source: :run_id" lib/`
is empty, so x1's transitional column mapping is gone as sp-j2y removed
it; the promoted enumeration carries the `KeyGenerator.table/0` row and
the UXID `exec` prefix row.

No tag, no publish - both are the operator's.

Refs: sp-8b7