* test: raise Trax.Core line coverage from 86% to 97%

Cover Monad null-service throw paths across all 7 AddServices arities,
the protected Chain/IChain/Extract/ShortCircuit/AddServices wrappers on
Train (reachable only via overridden Junctions()), the MonadTask async
wrapper surface, InitializeJunction error paths, ExtractTuple arities
2-7, ExtractLoggerFromLoggerFactory branches, and the Chain/ShortCircuit
early-return guards (preset Exception, missing input, multi-ctor
junction, unregistered IChain interface).

* docs: add coverage badge to README

* docs: switch to live Codecov badge

#20)

*.csproj.lscache is produced by MSBuild's static graph restore and should
not be checked in. Generated locally on every build/restore.

Add a per-repo Tests.Meta project (Tests/ for Trax.Docs) that enforces
CLAUDE.md conventions in CI: locked Directory.Build.props version, no
[Ignore], no legacy Assert.*, no fixed Task.Delay (with baseline
grandfathering), cross-repo Version="1.*", PascalCase test folders,
AddTrax*/UseTrax* naming, and a PublicApiGenerator surface snapshot per
assembly.

Trax.Effect also gets migrations-integrity + Model<->Persistent pairing
checks. Trax.Mediator gets a runtime interface-FullName invariant test and
a DI composition smoke test. Trax.Scheduler moves
SchedulerConfigurationBuilder.Build into its own partial file to comply
with the builder convention. Trax.Docs gets em-dash / Jekyll /
link-resolves / SDK-Reference-block lints and replaces 21 pre-existing
em-dashes.

Failure messages name the CLAUDE.md section and the offender. Pre-existing
violations are grandfathered via per-test allowlists (BaselineOffenders,
KnownExceptions, KnownBrokenLinks) so CI stays green; new regressions
fail.

Hand-maintained and not consumed by the release pipeline (no @semantic-release/changelog plugin).

* Add Trax.Core.Testing: reusable architecture-guard infrastructure and hygiene checkers

Framework-agnostic guard checkers (RepoRoot/SourceFiles/SourceText, ArchitectureGuardOptions, GuardResult) plus hygiene/repo-convention checkers (no [Ignore], no legacy asserts, no fixed delays, Directory.Build.props version, cross-repo Version=1.*). No test-framework dependency: each checker returns an offender list a consumer asserts on with its own framework. 11 unit tests.

* Ship NUnit base fixtures: subclass + supply config, no test bodies

Each guard becomes an abstract [TestFixture] with the [Test] methods pre-written; consumers subclass it, override the config (options, types, assemblies), add [TestFixture], and run dotnet test. The framework-agnostic checker methods remain for non-NUnit use.

* Cover the guard fixtures and infrastructure with tests

Adds self-test fixtures that run each base fixture as a consumer would (subclass + configure against a synthetic repo), plus direct tests for RepoRoot resolution, SourceFiles build-output exclusion, and the RepoConventionGuards missing-file / unparseable-project branches. Closes the coverage gaps Codecov flagged on the new code.

Release trigger: the originating change (#24) squash-merged with a non-conventional
title, so semantic-release classified it as a non-release and no package was published.
Merging this lands a feat: commit on main, cutting the release that publishes
Trax.Core.Testing (and the other Trax.Core packages at the bumped version).