Add a Compose Multiplatform desktop app for Windows, Linux and macOS

[solcott]

Adds :desktop, the JVM counterpart to :app and :web. It builds ComposeGraph, reads circuit off it, and mounts the same CountriesApp.

Most of this was already paid for: CountriesApp was extracted for :web, every library module already publishes a jvm target, and :network already had a JVM platformConfiguration pointing the Apollo SQLite cache at ~/.apollo. What is genuinely new is a main(), a Window, two platform affordances, and the packaging.

Commits

c96678a	:ui gains LocalFlagFontFamily — null by default, so a no-op everywhere today
b5fb926	the module: build config, main(), window, icons
f29df6c	keyboard back, which is what forces the backstack out of CountriesApp
f9c4f4c	the bundled flag font and its OS guard
962cc07	AGENTS.md, DECISION_LOG.md, README.md

Each compiles and passes ktfmtCheck on its own.

Decisions worth a look in review

A plain kotlin("jvm") module, not multiplatform. Desktop is the jvm target, so kotlin { } would hold exactly one target and src/jvmMain would differ from src/main only in name. :web earns multiplatform because it serves js and wasmJs from one module.

compose.desktop.currentOs is the one dependency declared through a plugin accessor rather than a catalog coordinate. It has to be — skiko's runtime jar is classified by OS and architecture. The consequence is that everything built here runs on the build host's OS only, uber jar included. Real cross-platform installers need a CI matrix, which is deliberately not in this PR.

nativeDistributions { modules(...) } is load-bearing and fails invisibly. jpackage jlinks a trimmed runtime that has neither java.sql/jdk.unsupported (sqlite-jdbc) nor java.naming/jdk.crypto.ec (OkHttp TLS). run uses the full JDK, so a missing entry only surfaces in an installed build, as a crash on the first query.

onRootPop stays the default no-op. Android passes finish() because that is what back-past-root means there; on desktop the close button is how you leave, and Esc on the list should not quit the app.

The flag font, which is where the real work was

Skia has no system font manager, so desktop draws with whatever the OS provides:

	Flags	Non-Latin native names
macOS	fine	fine
Windows	letter pairs — Segoe UI Emoji has no flag glyphs, by Microsoft's policy	fine
Linux	tofu without Noto Color Emoji	tofu without Noto CJK

So the app carries NotoColorEmoji-flagsonly.ttf, upstream verbatim (OFL 1.1, notice committed beside it). Two constraints, both found by rendering rather than by reasoning:

• It must be the CBDT build, not COLRv1. COLRv1 needs FreeType 2.11+ or a Windows 11-era DirectWrite, and where unsupported it draws nothing rather than falling back. Blank is a worse failure than letters.
• It must not reach macOS. Skia goes through CoreText there, which refuses a bitmap-only font outright — makeFromData returns null — so the flags would vanish on the one platform that never needed the font. The COLRv1 build is not the escape hatch: CoreText loads it and then renders its layers as nothing. needsBundledFlagFont() is the guard.

The second one is the instructive failure. It loaded, it shaped the ligature, it reported a sensible advance — every signal short of the pixels said it worked. A control render through identical code (Apple Color Emoji: 697 distinct colours; the bundled font: 1) settled it.

Verification

• 14 desktop tests, root test, and ktfmtCheck all pass.
• assembleDebug and :web:jsBrowserDistribution still build — the :ui change touches every platform.
• :desktop:run launches clean.
• The .dmg was built and the installed app exercised. With ~/.apollo/countries.db moved aside, the packaged build on the jlink-trimmed runtime fetched over HTTPS and wrote a fresh cache with 258 records. That is the only run that proves the modules(...) list.

Not verified: Windows and Linux. I have neither, and the flag font is precisely the thing that only shows itself there. FlagFontTest states the invariant as far as a Mac can — "wherever Skia can load this font it must ligate and rasterise in colour, and where it cannot, the app must not be using it" — plus a structural check that the file is still the CBDT build. That is a real regression guard, and it is not the same as having seen a flag on Windows.

Worth knowing for a follow-up: that test takes the trivial branch on macOS and becomes a real colour assertion on windows-latest and ubuntu-latest. A CI matrix would close this gap for free, and Skia's raster rendering needs no display.

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