Two long-standing gaps in the architecture documentation are
filled in this commit:
1. doc/architecture/postit.md is new. It covers everything the
existing postit-oidc.md does not: the one-codebase /
three-frontends topology (PostIt lib + PostIt.Desktop +
PostIt.Android + PostIt.Browser), the custom ViewLocator
that resolves ViewModel -> View through the DI provider
(and why we don't use the Avalonia.Mvvm default), the
composition root in App.OnFrameworkInitializationCompleted
with the full DI registration table, the navigation flow
driven by SessionStatusViewModel events, the ViewModel
lifetime conventions (singleton vs transient), the
[RelayCommand] XAML binding conventions (referenced to
AGENTS.md for the canonical version), and the per-page
DataContext / role table. The Settings-singleton invariant
is called out as a guard rail, and the SettingsPage
anti-empilement invariant is documented as the TODO the
code still owes us.
2. doc/architecture/decoupage-organisation.md is brought up to
date. Its project table listed 7 .csproj; the repo has 14
(the four PostIt projects, the tests satellites, the cli
tool). The table is extended, the ASCII diagram picks up
the PostIt block, and an Outils et tests section lists
the test / CLI satellites that were missing.
doc/README.md is updated to index the new postit.md. No code
changes in this commit, no behaviour change.
