feat(c3): Containers log-follow ([f]) — live tail with pause/resume

[paulp83]

What

Adds a [f] log-follow (live tail) to c3's Containers view Logs drill: new docker logs lines appear every ~2 s without re-pressing [l], with pause/resume and a live state indicator.

How

• Poll, not stream — a 2.0 s interval re-reads the existing one-shot container_logs(name, tail=200) service call. No docker logs -f subprocess, no new service surface (services.py untouched).
• Anchor dedupe — after any full-tail render the anchor is the last displayed line; each tick appends only the lines after the anchor's last occurrence. Anchor missing (tail scrolled past 200 lines, restart, in-place progress-bar line) or container changed → resync: clear + full fresh tail.
• No interleaving — ticks share the exclusive container-logs worker group with stream_container_logs, so a tick and a manual [l]/navigation snapshot never interleave; cancellation races self-heal via resync.
• Moving underline — new lines render underlined; each tick the previous batch drops back to plain and only the newest batch keeps the underline. RichLog is append-only, so the tick keeps a chronological display model (plain / newest-batch / state-note) and re-renders the pane from it while the user sits at the tail; while reading history (scrolled up) ticks append plain with no rewrite, and the next at-tail tick re-renders correctly.
• State — three explicit states: off / following / paused. Pane title: Live / Live ● following (green) / Live … paused (yellow); pause/resume notes are display-only.
• Invariants — the [Y]-copy tail stays plain text (markup stripped by the existing buffer; notes use buffer=False); no background polling when leaving the Containers tab or mode 0 (disarm on leave, re-entry starts off); the followed container dying → death note + disarm; browsing to a different stopped row keeps the mode armed (tick no-ops).

tui-core

LivePane gains tail-reader scroll-follow for every consumer: scrolling up stops auto-scroll (history stays readable while lines stream in), scrolling back to the bottom resumes it — gated on both the append-time scroll_end and RichLog.auto_scroll (which otherwise yanks the pane on every write). Plus set_title() and an at_bottom property.

Tests

• 19 headless tests in TestContainerLogFollow (subprocess-free — injected FakeRunner, ticks driven by direct _log_follow_tick() calls, no real 2 s waits): binding/context gating, arm/pause/resume/disarm, tick dedupe + quiet-log no-op, resync, moving-underline batch tracking, note survival across re-render, scrolled-up plain append, navigation rebase, death/error disarms.
• tui-core scroll-follow + at_bottom tests.
• README keybindings row for f.

Verified on a live rig: full c3 suite green (896 passed; the two failures are a pre-existing registry-reps count mismatch that fails at pre-feature commits, and a tab-bar focus timing flake that passes in isolation), plus a read-only live smoke test driving the production app against real docker (arm → 3 real ticks on an actively-logging container → pause → resume → navigate to a quiet container → disarm-on-tab-leave).

Design spec: docs/superpowers/specs/2026-08-16-c3-container-log-follow-design.md.