feat(messages): tag-filtered feed, sharing the feed implementation (#29)

[Adron]

Closes #29. Also fixes #97. Part of epic #27.

The tag feed is not a fork

MessagesFeedViewModel reads the tag from SavedStateHandle and is hosted twice from the NavHost.
#22's opaque cursor paging, #19's view-preference switcher, digs, edits, moderation and
pull-to-refresh are the same code under a different query. observeFeed/refreshFeed/
loadMoreFeed each took a tag: String? defaulting to the main feed, so every existing call site is
unchanged. The tag goes to Retrofit raw, so it is percent-encoded exactly once.

The composer is the one thing a tag feed omits — a new post cannot be filed into someone else's tag —
which also lets it skip two composer-only network calls.

The Room cache was the real design problem

The main feed and a tag feed routinely contain the same message, and a message row cannot hold
two positions — so keeping feedOrder on the message meant the second feed to load would steal
rows from the first.

Feed membership moved to a feed_entry(feedKey, messageId, position) table over shared message rows
(feedKey = "" for the main feed, the tag itself otherwise; tags are never blank, so they cannot
collide). Refreshing a tag feed now clears only its own memberships; a dig/edit/delete in either feed
is seen by both; ON DELETE CASCADE drops a deleted message from every feed at once.

One sharp edge found and fixed while reviewing: @Insert(OnConflictStrategy.REPLACE) deletes
before re-inserting, which would have fired that cascade and silently evicted rows from the main
feed. insertAll is now @Upsert, with the reason recorded in both the DAO and the entity.

The deep-link URL is verified, not guessed

/tag/lists, /tags/lists, /tags, /explore and /trending all 404; /messages turned out
to be Direct Messages, not the feed. Pulling the home page's RSC payload and JS chunk surfaced the
web's own tag chip:

A.tags.map(e => jsxs(Link, { href: "/?tag=".concat(encodeURIComponent(e)), … }))

and https://interlinedlist.com/?tag=lists does serve a filtered feed — verified, including for
tag=life is short, o brave girl. So the canonical URL is
https://interlinedlist.com/?tag=<url-encoded tag>.

⚠️ One product decision worth a second opinion

The manifest now registers https://interlinedlist.com/. An intent filter cannot match on a
query string, so / is the narrowest filter that can catch /?tag=…. The consequence: a plain
homepage link will now also open the app, landing on its normal start destination — which shows the
same feed, so it is not broken, but it is a wider claim than the feature needs. Called out in a
manifest comment. Say if you would rather not claim it.

Also fixes #97

A newly created message was inserted at maxFeedOrder - 1 — near the bottom of a loaded feed
rather than the top, because the feed sorts ascending. It is now minPosition - 1, a genuine head
insert. The existing "at the top of the feed" test passed before only because it used a single-row
feed.

Verification

./gradlew :app:assembleDebug testDebugUnitTest → BUILD SUCCESSFUL, 1348 tests, 0 failures (234
in :feature:messages).

Tests cover: the tag on the wire verbatim; %20/%2C present but no %25, proving
single-encoding; tag alongside onlyMine; cursor paging on the filtered feed with the token verbatim
and no offset; the main feed untouched by a tag refresh; per-tag eviction isolation; cross-feed
dig and delete; 14 deep-link cases (web/www/http/custom-scheme, decode-once, other pages and
foreign hosts rejected, blank tag rejected, garbage → null). I also inspected the generated
MessageDao_Impl/MessagesDatabase_Impl to confirm the JOIN compiled, that message writes are
INSERT/UPDATE rather than INSERT OR REPLACE, and that the FK is created ON DELETE CASCADE
under PRAGMA foreign_keys = ON.

DB v5 → v6. Compose tests compile, not executed (no emulator).

Remaining risks

• No Room SQL is exercised at test time. The repo has no Robolectric and no DAO tests, so the
  join table is verified only by Room's compile-time query validation plus the fake.
• Orphan message rows: clearFeed now drops memberships rather than message rows, so rows that
  leave every feed linger. Bounded in practice, and the DB is fallbackToDestructiveMigration. A
  prune was deliberately not added, because the obvious one would delete a message the detail screen
  is currently showing.
• A deep link arriving while signed out is dropped, exactly as notification routes already are.

MessagesDestinations.tagFeedRoute(tag) is the single entry point #30 (trending) will call.

[Adron]

Merged into parity/queue. One conflict resolved on the way in: MainActivity.onCreate gained blog-link resolution from #88 (PR #121) and tag-link resolution here — independent additions, both kept. Re-verified after the merge: :app:assembleDebug testDebugUnitTest green.