fix(tui): preserve multi-click text selection

[kitlangton]

Why

The click-only selection guard introduced with #44971 clears selection on mouse release. OpenTUI also resets its multi-click counter when selection is cleared, so double- and triple-clicks never reach word or line selection when copy-on-select is enabled.

This caused the deterministic selection-test failures seen in #45157. That branch passed locally because it lacked the upstream guard, while CI tested the combined merge commit.

What Changes

Interaction	Result
Bare terminal click	Does not copy or reset click history
Double-click	Selects and copies the word, preserving its highlight
Triple-click	Selects and copies the line, preserving its highlight
Empty drag	Does not copy or reset click history
Explicit copy of an invalid selection	Still clears the invalid selection

Selection History

Stationary word and line selections can still report isStart = true. Only a start-only cell selection is treated as a bare click. Mouse-release copying ignores invalid selections instead of clearing them, preserving the renderer-owned click count without private-state manipulation.

Demo

Matched real OpenCode Drive recordings: base cbef698861 on the left and fix aa6297f7ae on the right. Both use the same fictional simulated response and native SGR mouse input through the real TUI. Before: double/triple clicks select nothing. After: word and line highlights remain visible and clipboard confirmation appears.

click-selection-before-after.mp4

Scope

Restores real double/triple-click behavior without weakening the existing dialog or selection tests. A bare embedded-terminal click retains its native single-cell highlight until another interaction clears or replaces it, but never auto-copies that cell. Explicit copy retains its existing invalid-selection cleanup.

Verification

# From packages/tui
bun run test test/cli/tui/dialog-select.test.tsx test/util/selection-copy-on-select.test.tsx test/util/selection.test.ts --rerun-each=20
bun run test
bun typecheck

# From repository root
bun run lint packages/tui/src/util/selection.ts packages/tui/test/util/selection.test.ts packages/tui/test/util/selection-copy-on-select.test.tsx
bunx prettier --check packages/tui/src/util/selection.ts packages/tui/test/util/selection.test.ts packages/tui/test/util/selection-copy-on-select.test.tsx

Focused tests: 560 passed across 20 repeats. Full TUI suite: 810 passed, 5 skipped, 0 failed. Typecheck, lint, formatting, and all 32 pre-push workspace typecheck tasks passed. Includes real embedded-terminal coverage for click suppression, nonempty drag copying/highlight preservation, and empty drags. Local validation was on macOS; Linux and Windows validation runs in CI.