Fix: alternate-screen scroll detaches buffer lines, breaking text selection

[lordspace]

Fixes #239.

The fix

Buffer.scrollUp shifts lines up a slot with lines[i] = lines[i + n], which
leaves a stale duplicate reference in the source slot. _adoptChild detached
whatever occupied the target slot unconditionally, so the next iteration
detached the line just re-homed one index earlier — after a single
alternate-screen scroll effectively every visible line was detached, and a
selection anchored to a detached line reads back as null.

This detaches the previous occupant only when the slot is still its home.
_attach already rewrites _absoluteIndex, so a re-homed item maps elsewhere
and survives, while an eviction on a full list still detaches exactly as before.

Full diagnosis, reproduction and measurements are in the issue.

Tests

• test/src/utils/circular_buffer_test.dart — shifting items up a slot keeps
  them attached, and the evicted item still detaches.
• test/src/core/buffer/alt_screen_selection_test.dart — scrolls the alternate
  screen, then asserts a real selection resolves and yields the right text.

The existing can track index of items test earned its keep here: a first
version of this patch passed the new test but broke that one by sparing an item
that was genuinely being evicted. Both pass now.

The two dependency commits

Drop the discontinued dart_code_metrics dev dependency is a prerequisite,
not housekeeping. The package was discontinued, and it pins test_api versions
that conflict with the current flutter_test — so flutter test cannot resolve
dependencies at all and no test in the package runs on a current Flutter
SDK. Its analyzer.plugins entry and config block go with it.

Upgrade lints to 6.x and refresh the dependency lock — lints 3 -> 6 was
the only constraint needing a raise (build_runner, mockito and test
already resolve to their ceilings under the existing ones). Two one-line changes
satisfy the new strict_top_level_inference rule: a return type on the
IndexAwareCircularBuffer index setter, and a parameter type on _onTapDown.

Happy to rebase down to the bug fix alone if you would rather take the
dependency work separately — the fix stands on its own.

Verification

flutter test — 115 passing. flutter analyze reports only pre-existing
SDK deprecations (withOpacity, OverlayPortal.targetsRootOverlay).