Inject ScrollbackStore and ScrollbackIndex so tests can't pollute scrollback either

[buggerman]

Summary

• Stacked on top of Add scrollback recovery script for the test-pollution incident #35 (recovery script). Make ServerStore instance-based so tests can't poison production servers.json #34 stopped servers.json overwrites; this finishes the job by stopping IRCSessionTests and AppStateNavigationTests from creating fresh orphan scrollback dirs and trickling test-fixture rows into the production FTS5 index on every swift test run.
• Same injection pattern as Make ServerStore instance-based so tests can't poison production servers.json #34: IRCSession and AppState accept ScrollbackStore and ScrollbackIndex via init with no defaults. Production passes .shared (path unchanged). Tests pass tempdir-rooted instances.
• Read-only ScrollbackIndex.shared.search from ContentView is left alone — it's the production search read path, never hit by tests.

Changes

• Sources/BryggaCore/IRC/IRCSession.swift — adds private let scrollbackStore and private let scrollbackIndex. Constructor becomes init(server:connection:scrollbackStore:scrollbackIndex:). Routes record(_:in:), recordServer(_:), loadScrollbackIfNeeded(for:), and finalizeChathistoryBatch through the injected instances. Stores are captured into local vars before Task blocks to avoid strong-self capture.
• Sources/BryggaCore/Models/AppState.swift — adds scrollbackStore and scrollbackIndex properties. Constructor becomes init(store:scrollbackStore:scrollbackIndex:). restoreFromStore's rehydrate task, closePrivateMessage's clear, and removeServer's clear all go through the injected instances. addServer(...) plumbs them through to every IRCSession it constructs.
• Sources/Brygga/BryggaApp.swift — AppState(store: .shared) → AppState(store: .shared, scrollbackStore: .shared, scrollbackIndex: .shared).
• Tests/AppStateNavigationTests.swift — replaces makeStore() with makeDeps() returning a (ServerStore, ScrollbackStore, ScrollbackIndex) triple, all rooted under a unique tempdir. Updates all three AppState(...) construction sites.
• Tests/IRCSessionTests.swift — makeSession(...) now constructs both stores per-call (tempdir + :memory: SQLite) and passes them to IRCSession.

Test plan

• swift build — passes
• swift test — 132 tests pass
• swiftformat --lint . — clean
• Manual verification that swift test no longer touches the prod scrollback paths:

  SCROLLBACK="$HOME/Library/Application Support/Brygga/scrollback"
  SQLITE="$HOME/Library/Application Support/Brygga/scrollback.sqlite"
  echo "before: $(find "$SCROLLBACK" -mindepth 1 -maxdepth 1 -type d | wc -l) dirs, $(stat -f %z "$SQLITE") bytes"
  swift test > /dev/null
  echo "after:  $(find "$SCROLLBACK" -mindepth 1 -maxdepth 1 -type d | wc -l) dirs, $(stat -f %z "$SQLITE") bytes"

  Verified locally: dir count and DB size unchanged (246 dirs, 475136 bytes both before and after).

Risk / rollback

• API change: IRCSession.init(server:connection:) is removed. AppState.init(store:) is removed. Any external consumer passing only (server, connection) or only (store:) would need to update. Internal call sites all updated in this PR.
• Default .shared instances keep prod behavior identical — same JSONL paths, same SQLite path, no migrations.
• Revert: this single commit. Reverting reintroduces test pollution but doesn't break anything else.

Stacking note

Per the squash-merge stacked-PR rule for this repo: merge this PR first (lands both this and the recovery script onto main as one squashed commit), then close #35 — its content has already landed.