build(hygiene): ship symbols, gate warnings, align casing (#121)

[mrcsin]

Closes #121.

Problem

Four build-infrastructure gaps, plus the deployed paths that carried the same defect.

• Release shipped no symbols. DebugSymbols=False and DebugType=None meant every stack
  trace in the Serilog file landed without a file or line number. The log was never empty —
  exceptions are passed as objects and the template renders them, so type, message and the
  frame chain all arrived — but the in <file>:line <n> suffix did not, and on a long trace
  through the sync executor knowing the method is not enough to know which branch threw.
• Nothing enforced the zero-warning state, so it could rise silently.
• InternalsVisibleTo was declared two ways — the SDK item in Core, a raw
  AssemblyAttribute in UI.
• The assembly was named Semistep while the product and every namespace are SemiStep.
  The deployed C:\DISTR\Config\ and C:\DISTR\Logs\ roots carried the same casing.

What changed

• DebugType=portable for Release. The installer picks the PDB up on its own — it copies the
  publish directory whole and excludes nothing.
• TreatWarningsAsErrors and EnforceCodeStyleInBuild set centrally. NuGet audit codes
  NU1900-NU1904 are exempt: an advisory published against a transitive package, or an
  unreachable audit feed, would otherwise fail CI with no change in the repository.
  AnalysisLevel is deliberately absent for the same reason — global.json rolls forward
  across feature bands, so latest would move on its own.
• dotnet_diagnostic.IDE1006.severity = warning in .editorconfig. This is what makes the
  style gate actually bite: EnforceCodeStyleInBuild alone does not, because the
  dotnet_naming_rule severities drive the IDE rather than the build. It matters because CI
  runs no dotnet format step and the pre-commit hook is not versioned, so a fresh clone had
  no style enforcement at all.
• One spelling of InternalsVisibleTo, matching Core.
• AssemblyName → SemiStep, with the eleven avares:// URIs and one assembly= reference
  that resolve against it, the installer's AppExeName, .zed/debug.json and the localization
  doc. The deployed roots are recased in StartupOptions and in the installer.
• The six [InstallDelete] entries clearing the legacy flat config layout are removed rather
  than recased — no machine carries that layout, so the cleanup has nothing left to do.
• .github/workflows/ci.yml paths gains SemiStep.slnx and .editorconfig; both now define
  the build and a change to either alone would not have triggered CI.
• New Docs/architecture/build-and-deployment.md records the gate, the audit exemption, the
  IDE1006 finding, and the fact that StartupOptions.cs and Installer/SemiStep.iss duplicate
  the deployed literals with nothing checking that they agree.

Verification

• Clean Release build: 0 warnings, 0 errors
• dotnet test SemiStep.slnx — 1444 passed, 0 failed
• dotnet publish emits SemiStep.pdb alongside SemiStep.exe
• ISCC.exe compiles the installer with the new SrcCfgDir / ConfigRoot / LogRoot defines
• Both gates proven to bite: an unused local fails with CS0219, a private field violating the
  naming rule fails with IDE1006
• The rename was the risk — the headless suite boots the real App and loads
  avares://SemiStep/Styles/ColorPalette.axaml, so a broken rename would have failed all 615
  AvaloniaFact tests as a group. It did not.

Note for reviewers

The C:\DISTR rename is safe on an existing installation because NTFS is case-insensitive:
Config\Semistep\ and Config\SemiStep\ are the same directory, so an upgraded machine keeps
reading its configuration and the installer overwrites in place. Nothing is stranded.

The deployed literals live in two places by hand — StartupOptions.cs and
Installer/SemiStep.iss — and nothing at build time checks that they agree. The only detection
is a manual install. That is documented rather than fixed here.