fix(viewer): keep tail-first follow attached

[siriusctrl]

Summary

• keep tail-first live timelines attached on their first rendered frame when pretty JSON adds a sticky breadcrumb
• preserve the existing one-draw tail guard for snapshots and paused views while giving active follow and explicit reattach exact-tail pins precedence
• add backend-neutral coverage for wrapped, unwrapped, narrow-height, older-prepend, append, detach, reattach, pause, and resume behavior
• document the user-visible fix under Unreleased

Root cause

A tail-first viewer begins with both preserve_tail_on_next_draw and FollowState::Following. During sticky-layout convergence, the exact live tail was computed and pinned, then the older one-draw preserved position overwrote it. When the JSON breadcrumb changed the visible height, the viewport ended one row before the recomputed tail and the render state immediately became follow:detached.

The fix makes active follow or explicit reattach tail pins authoritative. Paused follow and ordinary snapshot preservation are unchanged.

Validation

• regression first failed on the 0.6.1 implementation with follow:detached, then passed with the fix
• cargo fmt --all --check
• cargo check --workspace --all-targets --locked
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --all-targets --locked: 384 passed, 0 failed, 13 ignored
• record timeline suite: 60 passed
• load performance smoke, growing-record-stream/follow, 3 samples: median 3.025 ms
• real 120x34 PTY smoke using the release binary showed the committed tail marker with follow:on and clean terminal exit

PTY evidence remains under ignored target/ and is not part of the PR. No release, tag, or publish action is included.