chore: finish CPM and move shared properties to Directory.Build.props

[StuartMeeks]

What changed

PR C of the NextIteration.Standards adoption sequence, and the last one. Closes §1.2 and §1.3, taking this repo to 27/27 machine-checkable clauses. §2.6 was already closed in PR A — the collector was referenced before that, but nothing passed --coverage, which is the difference between coverage collected and coverage listed in the dependency graph.

• Directory.Packages.props gains CentralPackageVersionOverrideEnabled=false.
• Directory.Build.props takes the fifteen properties both csprojs restated identically.
• Each csproj now carries only what is specific to it.

Why

The override guard is the half of §1.3 that does the work. Without it MSBuild silently ignores a stray inline Version= and uses the central one — the exact drift the file exists to prevent, prevented without ever saying so. Verified rather than assumed: adding Version="0.50.0" to a PackageReference now fails restore with NU1008.

Fifteen copies of a property is fifteen chances for one to drift, and nothing fails when a csproj quietly disagrees with its sibling about DebugType. The test project keeps GenerateDocumentationFile=false as an explicit opt-out — fixture code has no XML docs and TreatWarningsAsErrors would otherwise fail the build over every missing one.

EnforceCodeStyleInBuild is deliberately absent. §1.2.1 is blocked.

Consumer impact

None, and this is the one step where that is demonstrated rather than asserted. Both sides packed at this commit from a clean obj/, per ADOPTING.md — a reused obj/ makes the two builds embed different git SHAs via SourceLink, which moves the MVID and the deterministic PE stamp, and both assemblies then "differ" for reasons that have nothing to do with CPM.

Package entry	Before vs after
.nuspec	sha256 identical
lib/net8.0/…dll	sha256 identical
lib/net10.0/…dll	sha256 identical
lib/net8.0 + net10.0 .xml	sha256 identical
README.md, icon.png	sha256 identical
[Content_Types].xml	sha256 identical
.psmdcp content	identical
.psmdcp filename	differs — NuGet regenerates it per pack
_rels/.rels	differs only in that filename

That is exactly the one difference ADOPTING.md names as acceptable, and nothing else.

No public API, target framework, dependency floor or on-disk format change. Versions were copied across unchanged.

Checklist

• Build is clean — no new warnings (TreatWarningsAsErrors is on)
• Tests pass on every shipped target framework — 64/64 on net8.0 and net10.0, coverage collected on both
• Public API changes carry XML docs — no code changes
• CHANGELOG.md updated under [Unreleased]
• Dependency floors unchanged

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