feat(view): expose record timeline embedding facade

[siriusctrl]

Summary

• expose a narrow fmtview::view embedding facade around backend-neutral RecordTimeline sources
• keep JSONL selection, crossterm polling, terminal setup/cleanup, and frame commits inside the root fmtview package
• separate non-refreshing tail-first snapshots from live follow mode while preserving lazy older loads
• let backward structure navigation cross an unloaded older-record boundary
• add a complete embedding example, downstream dependency compile test, API/headless regressions, and maintainer/user documentation

Why

Fiasco and other applications need to inspect committed record timelines without depending directly on fmtview-core or reimplementing terminal lifecycle. Snapshot inspection must open at the committed tail and load history lazily, while live inspection needs the existing attached/detached/paused follow behavior.

Public contract

fmtview::view::run(
    source: Box<dyn fmtview::view::RecordTimeline>,
    options: fmtview::view::ViewOptions,
) -> fmtview::view::Result<()>

The facade fixes the view to JSONL and re-exports the complete timeline source vocabulary. ViewOptions::default() is a stable snapshot; follow = true enables refresh and follow controls.

Validation

• cargo fmt --all --check
• cargo test --workspace --all-targets --locked
  • includes an external temp crate that depends only on fmtview
  • includes 50 record-timeline tests covering snapshot/follow, search, structure navigation, reset, raw records, torn tails, and million-record bounded work
• cargo clippy --workspace --all-targets --locked -- -D warnings
• RUSTDOCFLAGS='-D warnings' cargo doc --workspace --no-deps --locked
• cargo package -p fmtview-core --list --allow-dirty
• cargo package -p fmtview --list --allow-dirty
• cargo publish -p fmtview-core --dry-run --locked --allow-dirty
• real script PTY smoke of the built embedding example using r, j, r, q
  • child exit 0 and shell state restored
  • alternate screen, mouse modes, synchronized updates, cursor restore, and cleanup ordering verified
  • raw and structured views both observed
  • evidence: target/fmtview-embedding-pty/pty.log and checks.txt in the implementation worktree

Packaging note

The root fmtview publish dry-run reaches tarball verification and then resolves the already-published crates.io fmtview-core 0.6.0, which does not contain this PR's new snapshot constructor. This PR intentionally does not change versions or publish. Per docs/releasing.md, the release that includes this change must bump both packages, publish the matching fmtview-core first, wait for registry availability, and then run the root fmtview dry-run.