refactor(sdk): drop unused incrementedUnread payload from ChatMessageAppendOutcome

[variablefate]

Summary

Stacked on top of #118 (renumber ADR). When #118 merges, GitHub will rebase this onto main.

The `.inserted(incrementedUnread:)` payload was a hypothetical-haptic-decision feature that turned out unused. `ChatCoordinator.handleChatEvent` only matches `case .inserted` — it never reads the payload. Tests that exercised the unread side effect also assert `store.unreadCount` directly, which is the canonical source.

Per CLAUDE.md's "Don't design for hypothetical future requirements" guidance, drop the payload. The enum becomes:

```swift
public enum ChatMessageAppendOutcome: Equatable, Sendable {
case duplicate
case inserted
}
```

First-principles check

The principle: state changes are observable through their canonical source, not via return-value side channels. `unreadCount` IS the canonical source. The payload was a side channel duplicating it. Dropping it eliminates the duplication without losing information.

If a future consumer (driver app, badge-pulse UI, push routing) genuinely needs a "did this append bump unread" signal that's race-free from polling `unreadCount`, the payload can be added back at that point with the actual use case driving the shape.

Changes

• `ChatMessageStore.swift`: enum simplified; `append(_:)` returns `.inserted` unconditionally on insert (the unread update logic itself is unchanged).
• `ChatMessageStoreTests.swift`: every `.inserted(incrementedUnread: ...)` rewritten to `.inserted`. `unreadCutoffSuppressesReplayedHistory` gets an explicit `store.unreadCount == 0` check after the stale append so the suppression assertion isn't implicit through the final-count math.
• `decisions/0020-chat-message-store.md`: Decision item 5 reframed; Rationale's "Append outcome enum" section rewritten to reflect the actual motivation (readable named cases at call sites) and document why the payload was dropped; Alternatives-Considered gets a new bullet covering the dropped payload.

Test plan

• `swift test --filter ChatMessageStoreTests` — 21/21 pass
• `xcodebuild test -scheme RoadFlareTests` — TEST SUCCEEDED (all app + SDK tests)
• Confirmed `ChatCoordinator.handleChatEvent` haptic semantics unchanged: `if case .inserted = store.append(message), !message.isMine` matches the new enum shape identically

No protocol-surface impact (Kind 3178 wire format unchanged).

[variablefate]

Code review

One issue found and fixed in 1b050d1:

• ADR-0020 Rationale cited "the project's YAGNI guidance" as if `.claude/CLAUDE.md` contained a YAGNI principle, but that file's Project Conventions cover SDK/app split, build verification, and concurrency only — no YAGNI section. Reworded the justification to stand on its own ("the payload was dead public surface") so a future reader chasing the citation does not come up empty.

Other checks clean: all call sites swept (no remaining `incrementedUnread` or `.inserted(` patterns), `ChatCoordinator.handleChatEvent` pattern-match still compiles against the no-payload `.inserted`, test semantic coverage preserved (the explicit `unreadCount == 0` after the stale append in `unreadCutoffSuppressesReplayedHistory` is strictly equivalent to the dropped payload-based assertion), ADR-0020 internal consistency confirmed.

roadflare-ios/decisions/0020-chat-message-store.md

Lines 119 to 130 in 1b050d1

	**Append outcome enum.** The enum's named cases (`.duplicate`, `.inserted`)
	read more clearly at call sites than a `Bool` would — the haptic decision
	in `ChatCoordinator.handleChatEvent` becomes a self-describing
	`if case .inserted = store.append(message), !message.isMine`. An earlier
	revision attached an `incrementedUnread: Bool` payload to `.inserted`,
	motivated by a hypothetical caller that needed to know whether the badge
	just bumped (e.g. for a "fresh-unread pulse" UI). The actual coordinator
	never read the payload — it only checks the case — and every test that
	exercised the unread side effect also asserts `store.unreadCount` directly,
	which is the canonical source. The payload was therefore dead public surface
	and was dropped; if a future consumer genuinely needs a fresh-unread signal
	it can be added back as an associated value at that time.

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