feat(data): retain conversations on archive() + expose archived filter + seed (#93)

[ilmoniemi]

What

• Conversation gains an archived: Boolean = false field (default keeps every existing construction site source-compatible).
• FakeConversationRepository.archive(id) flips from removing the conversation to setting archived = true via state.update { … }. Throws IllegalArgumentException for unknown ids (unchanged contract); idempotent on repeat calls.
• ConversationFilter gains Archived. Filter matrix:
  • All — everything (no predicate)
  • Channels — isPromoted && !archived
  • Discussions — !isPromoted && !archived
  • Archived — archived
• One archived discussion seeded (seed-discussion-archived, lastUsedAt = 2026-04-15T12:00:00Z) so the archived stream is non-empty for manual verification.

Issue

Closes #93. Unblocks the Archived Discussions UI split from #77. unarchive() is intentionally out of scope (sibling ticket).

Testing

• ./gradlew test ✓
• ./gradlew lint ✓
• ./gradlew assembleDebug ✓
• ./gradlew connectedAndroidTest — not run (no device/emulator connected; this is pure data-layer code with no instrumented coverage needed).

Test changes in FakeConversationRepositoryTest:

• Bumped All seed assertion 5→6 and added Archived = 1 assertion.
• Bumped createDiscussion_appearsIn_observeConversations_All from 6→7.
• Rewrote archive_removes_from_observeConversations → archive_movesConversation_from_Discussions_to_Archived_andRetainsInStore per AC.
• Extended seededDiscussions_remainEmpty id list to include the new archived seed.
• New tests: seededArchivedDiscussion_appearsIn_Archived_butNotIn_Discussions, archive_onUnknownId_throws, archive_isIdempotent.

ChannelListViewModelTest.stubRepo gains an Archived -> TODO("not used") branch to keep the when exhaustive. DiscussionListViewModelTest stubs don't switch on the enum and are unchanged.

Architecture compliance

Follows docs/specs/architecture/93-…md verbatim: new enum variant rather than a parallel method; !archived added to Channels and Discussions for symmetry; archive() preserves lastUsedAt and isPromoted; no ConversationId value type introduced; unarchive() deferred.

🤖 Generated with Claude Code

[ilmoniemi]

Code Review: #93

Decision: PASS

Findings

None.

Summary

Clean, spec-compliant implementation of the XS data-layer slice.

• Conversation.archived: Boolean = false keeps every existing construction site source-compatible (no other call sites needed updates — verified via grep across .copy(...) and named-arg Conversation(...) sites).
• ConversationFilter.Archived added; predicates correctly tightened: Channels -> isPromoted && !archived, Discussions -> !isPromoted && !archived, Archived -> archived. All remains unfiltered.
• archive() now flips the flag via state.update { ... }, preserving the IllegalArgumentException contract on unknown ids and behaving idempotently (no duplicate emission).
• New seed seed-discussion-archived with lastUsedAt = 2026-04-15T12:00:00Z (intentionally older than live discussions, per spec).
• seedDiscussion(...) gained a defaulted archived: Boolean = false param — keeps the seed list readable and avoids a near-duplicate helper.
• Tests: counts bumped (All 5→6, post-create All 6→7, Archived 1); the rewritten archive_movesConversation_from_Discussions_to_Archived_andRetainsInStore covers the full pre/post matrix (in Discussions, not Archived → not in Discussions/Channels, in Archived, retained in All); new coverage for the archived seed's three-stream visibility, unknown-id throw, and idempotency. seededDiscussions_remainEmpty extended to include the new seed.
• ChannelListViewModelTest.stubRepo gained the exhaustive ConversationFilter.Archived -> TODO("not used") branch. DiscussionListViewModelTest and erroringRepo stubs don't switch exhaustively on the enum, so no update needed there — correctly left alone.
• ./gradlew test and ./gradlew lint both pass locally. Single pre-existing deprecation warning in DiscussionListScreen.kt:165 is unrelated to this PR.
• data/ remains free of android.* imports — Compose-Multiplatform-safe.

The architecture spec is also committed at docs/specs/architecture/93-archived-conversation-retention.md. The sibling unarchive(...) ticket is correctly deferred.