docs: split the README into topic pages and move the changelog out

[CaffeinatedCoder]

The root README.md was 584 lines, 350 of them the PostgreSQL section alone — the landing page was mostly a provider manual for one of three packages. This splits it, following the arrangement in the CodoMetis.ValueRanges repository.

The split

File	Lines	Was
README.md	176	584
docs/postgresql-indexes.md	148	—
docs/postgresql-constraints.md	212	—
docs/sqlserver.md	47	—
CHANGELOG.md + 3 package changelogs	59 / 67 / 61 / 37	inline in 4 READMEs

PostgreSQL splits in two rather than into one page because that is the seam that already exists: indexes go through the differ plus the custom generator, while temporal and exclusion constraints are rendered as design-time SqlOperations.

No docs/architecture.md — that prose lives in CLAUDE.md and ClaudeMdConsistencyTests guards it. A second copy is drift with no guard.

The changelog move

It was the only part of the README that grew unboundedly, and it had two consumers pinned to its old home: release.yml's awk extraction and PackagingConventionTests' baseline lookup. Both now read CHANGELOG.md.

ChangelogConsistencyTests now asserts the ## x.y.z heading style rather than merely parsing it. release.yml matches that heading literally to build the release notes, so a section demoted to ### would still read as documented while the release job published a blank release. Verified locally: the workflow's actual awk extracts 6 lines of notes for 5.0.3 from the new file.

Two new guards

The split introduces a dependency on links, which is the one defect that renders perfectly.

DocumentationLinkTests — relative links resolve to real files, #anchors to real headings, and the packed READMEs under src/ carry no relative links at all. nuget.org renders PackageReadmeFile with no base URL, so a relative link is dead exactly where most consumers arrive, and nothing at pack time notices.

DocumentationApiTests — every method the docs name exists in the public surface (EF Core/Npgsql/BCL calls are an explicit allowlist), and no provider page names another provider's exclusive API. Package validation already fails the pack on a removed public member, so what this adds is the rename fixed in source and forgotten in prose, and the name simply typed wrong.

Changelogs are deliberately out of scope for the API test: an entry saying 5.0.0 shipped HasExclusionConstraint stays true after a later rename, and asserting over them would turn every rename into pressure to rewrite history.

Verification

DocumentationApiTests went in green, and the vacuity check is why it is worth anything. Emptying its allowlist — which must then report exactly the external calls — showed AddDbContext missing from the result, because \b[A-Z][A-Za-z0-9]*\( never matches a generic invocation. HasTemporalForeignKey<Subscription>( is how every generic API in these docs is written, so the entire generic surface was unchecked while the test reported green. The regex now takes an optional type-argument list.

Every guard was then broken deliberately and caught by the right test:

Break	Failing test
Renamed docs/sqlserver.md	relative links resolve
Reworded a root heading that docs/ links into	anchors resolve — named all 4 links
Relative changelog link in a packed README	packed READMEs are absolute-only
Demoted ## 5.0.3 to ###	root changelog documents the shipped version
Left a ## 5.0.3 copy in the root README	no README carries a changelog
Renumbered a package section out of order	sections are newest-first
Deleted a package CHANGELOG.md	every package has a changelog
HasExclusionConstraint typo'd	documented API exists
HasTemporalForeignKey<Subscription> typo'd	same — would have passed before the regex fix
HasExpressionIndex on the SQL Server page	provider pages stay in scope
HasExclusionConstraint in the core README	same, reported as "(a satellite)"

Deleting a package changelog initially crashed four unrelated tests with FileNotFoundException and buried the actionable message, so both classes now skip a missing changelog and let the dedicated test own it.

Unit suite: 202 passed. dotnet pack still puts a README in all three nupkgs (no NU5019), with the changelogs correctly not packed. Integration not run locally — needs Docker.

🤖 Generated with Claude Code