[nedtwigg]

A cleanup pass over #1280 and #1281.

setSelection used to work out whether a write was the same selection from its null-ness, plus a preserveProgramCopy flag. It now takes a write kind: replace, edit (a nudge), or follow (scroll, trim, or reflow).

• A pending copy reports its result for an edited or followed selection, but not for a replaced one.
• Only a follow keeps the program's copy offer.
• A per-pane counter, bumped on clear and on replace, replaces the identity object. beginDrag no longer sets identity itself.

When older scrollback is trimmed, watchSelection now follows the selection without re-reading its baseline or disposing and re-registering markers on every shifted render. The markers have already moved with the content, so this saves that work while output streams into a full buffer.

• The scroll marker no longer keeps a separate row; the row comes from the selection.
• markRow puts the cursor-relative marker offset in one place, shared with the reflow anchor.
• The followed span keeps the existing reflow anchor. A resize after the top row of its logical line is trimmed cancels the selection, as §3.4 requires.

Tests:

• The trim test now checks that the markers survive. The newer-selection copy test now also covers a direct replace. Each fails when its fixed line is deleted.
• The new lifecycle test uses COPY_FLASH_MS instead of a hard-coded 1500.

Not changed:

• Unifying the scroll marker with anchorSelection. That would mean running followReflow's line walks on every render, and the two answer different questions.
• A translate-only path for the copy editor's rebuild on each trim. It is more machinery than the saving justifies.
• A shared deferred-clipboard test helper. The fixtures module is shared with stories and has no vi.

Validation: lib tsc -b, standalone tsc --noEmit, spec and Markdown lints, and the lib suite pass. The one exception is relay-origin.test.ts, which needs a built remote-lib-common in a fresh worktree and fails the same way on #1281's branch.

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