fix(secrets): close the pre-host hole in the store-path guard (0.6.1)

Found by the HistorianGateway adoption of 0.6.0, which probed the rejected paths
instead of observing a clean boot — a clean boot being equally consistent with the
validator not being wired at all.

An application that expands ${secret:} references before building its host composes
this subsystem in a throwaway ServiceCollection with no IHostEnvironment in it, then
migrates the store from that container. SecretsOptionsValidator resolves the content
root from IHostEnvironment and skips the under-content-root rule when it is absent, so
pre-host an absolute in-content-root path passed the only rule still applying, the
migrator CREATED the database, and the boot failed a second later once the real host
registered IHostEnvironment and the rule finally fired.

The observable result was a rejected boot that still left an empty store, with -wal and
-shm siblings, at the rejected path — the same artifact that made the MxGateway outage
read as "the database is there, it's just empty", which is precisely what this guard
exists to prevent. A relative path was unaffected because the absolute-path rule fires
with or without a content root; that asymmetry is what exposed it.

The mistake was conflating "no IHostEnvironment registered" with "no content root
exists". New AddZbSecrets overload takes the content root explicitly; the 3-argument
overload is unchanged and still falls back to IHostEnvironment, so this is additive.
SecretsOptionsValidator gains a string? constructor alongside the IHostEnvironment? one.

Reproduced first in an isolated harness before changing anything: a bare host rejects
both bad paths leaving no artifacts, and a bare ServiceCollection with no
IHostEnvironment migrates an in-content-root path to disk. Both are now regression tests,
the second paired with a positive control asserting the old behaviour still holds when no
content root is supplied — so the test is about the explicit content root rather than
about paths in general.

Also documents an upgrade note that applies to 0.6.0: a store seeded under the old
relative default is orphaned rather than migrated, and nothing signals it.

ZB.MOM.WW.Secrets/Directory.Build.props

@@ -5,7 +5,7 @@
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <LangVersion>latest</LangVersion>
-    <Version>0.6.0</Version>
+    <Version>0.6.1</Version>
     <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
     <PackageReadmeFile>README.md</PackageReadmeFile>
   </PropertyGroup>