feat(audience): validate required properties for reserved events at runtime

[nattb8]

Summary

Ticket: SDK-687

• Track(string, Dictionary) now validates required properties for purchase, progression, resource, achievement_unlocked, and milestone_reached, throwing ArgumentException if any are missing — previously it silently accepted these reserved events with no properties at all, bypassing every check the typed classes enforce.
• New internal IBuiltInEvent marker interface lets Track(IEvent) tell Immutable's own typed classes apart from a consumer's custom IEvent. Built-in events now throw straight through on a validation failure (matching Identify()/Alias()'s existing behavior for caller mistakes); a consumer's own custom IEvent is still caught and dropped with a warning, since a third-party bug shouldn't crash the game.
• New test reflects over every IBuiltInEvent implementation to catch a future one being added without a matching ReservedEvents.RequiredProperties entry.
• Purchase.Value changes from decimal to string, to avoid precision loss on very large or high-precision amounts.

	Before	After
Track("purchase", new Dictionary<string,object>())	Ships silently with missing fields	Throws, listing exactly which required properties are missing
Track(new Purchase{Currency="USD"}) (missing Value)	Caught by Track(IEvent) and warn-dropped	Throws straight through, matching Identify()/Alias()
Track(new SomeThirdPartyCustomEvent()) whose ToProperties() throws	Caught and warn-dropped	Unchanged, still caught and warn-dropped
Purchase.Value	decimal	string

Breaking changes to flag before release: existing code setting Purchase.Value with a decimal literal (e.g. 9.99m) will fail to compile. Existing code with a latent bug that sends an incomplete built-in typed event will now throw instead of silently warning — worth a major version bump and changelog callout, not a silent release.

Test plan

• dotnet build, dotnet test pass: 394 passed, 1 skipped (pre-existing), 0 failed
• dotnet format --verify-no-changes clean
• New/updated tests cover: string-overload missing required props throws per reserved event (including milestone_reached), all-required-props-present does not throw, unrecognised event names still unvalidated, a built-in typed event (Purchase) missing a required field now throws instead of warn-dropping, a genuinely custom IEvent whose ToProperties() throws is still caught and warn-dropped, every IBuiltInEvent implementation has a ReservedEvents entry

🤖 Generated with Claude Code

[github-actions]

Audience SDK — Build Size

Platform	SDK Size	Change
Android	0.40 / 20.00 MB	+0.04 MB	✅
Windows	0.17 / 20.00 MB	+0.01 MB	✅
iOS	8.61 / 20.00 MB	+0.09 MB	✅
macOS	1.00 / 20.00 MB	+0.10 MB	✅

SDK Size = build minus empty app. Change = vs baseline. Fails if any platform exceeds its absolute size limit.