feat!: route every lookup through a replaceable message formatter

[rent-a-developer]

Part of #29
Refs #30

Every lookup, with or without arguments, now passes its resolved value through ILocalizer.MessageFormatter (default: StringFormatMessageFormatter). A failed lookup raises ILocalizer.LookupFailed and returns the failure text from ILocalizer.LookupFailureTextBuilder. RefreshTriggeringRevision changes on every change that can change a lookup's result.

BREAKING CHANGE: a formatting failure no longer throws FormatException. Get returns the failure text (by default !key!) and raises ILocalizer.LookupFailed, and XAML renders the failure text. Get(key, args) with an empty array returns the resource value unformatted. ILocalizer has new members (MessageFormatter, LookupFailed, RefreshTriggeringRevision, a hidden Get overload) that custom implementations must add. TranslationNotFound and TranslationNotFoundEventArgs are replaced by LookupFailed and LookupFailedEventArgs, which also report formatting failures. MissingTranslationFormat is replaced by LookupFailureTextBuilder, a Func<LookupFailure, string> that also serves formatting failures. See docs/guides/migration-2.0.md.

Checklist

Everything under Always applies to every pull request. The rest applies only when its trigger does —
tick it, or leave it and say why in the description. A box that does not apply is not a box to tick.

Always

• pwsh -File scripts/pre-commit-gate.ps1 passes. It runs the public-API reminder, checks line endings,
  style, formatting and member ordering, builds Release and runs the test suites. Use -Fix to have it
  apply the tidying rather than only report it.
• The Release build produces zero warnings. TreatWarningsAsErrors is on, so this also covers style,
  member ordering, trim and public-API diagnostics.
• Branch name follows <type>/issue-<number>-<slug>, or <type>/<slug> when there is no issue — see
  CONTRIBUTING.md.

If the change adds behaviour or fixes a bug

• It is covered by tests.
• CHANGELOG.md has an entry under ## [Unreleased]. Do not bump a version — the version, the
  release date and the tag are the maintainer's, at release time.

If the change touches one UI package

• It is mirrored into the other, or it genuinely does not apply there — see
  CONTRIBUTING.md. The Avalonia and WPF markup
  extensions and converters are deliberate mirrors of each other.

If the change touches resource lookup, the generated keys, or the packaging that carries them

Culture fallback, satellite discovery, the source generator's output, the buildTransitive wiring, or a
dependency version. Nothing is trimmed on the just-in-time compiler, so no other check in this repository
can see the damage a mistake here does to a trimmed application.

• pwsh -File scripts/verify-package-aot.ps1 -Pack passes for both frameworks
  (-Framework net8.0 and the net10.0 default).
• No new IL2xxx / IL3xxx diagnostics, and none suppressed.

If the change touches the public API

• The affected PublicAPI.Unshipped.txt is updated with pwsh -File scripts/update-public-api.ps1, and
  the diff was reviewed line by line. A *REMOVED* entry is a breaking change.
• The XML documentation and the affected pages under docs/ are updated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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