tui: draw session tree rails in full mode only

[edwin-zvs]

Change

Tree rails become a full-mode affordance. Compact mode indents by exactly the
width the rails would have occupied, so hierarchy still reads and no column
downstream moves.

compact                          full
──────────────────────────       ──────────────────────────
  ● Prepare Patch Release          ● Prepare Patch Release
▼ ● Session Statuses ⑂2            – 14s ago – …
  ⑂ ● No Nested Session
  ⑂ ● Has A Nested Fork          ▼ ● Session Statuses ⑂2
     ⑂ ● Grandchild Fork           – 16s ago – …

                                 ├─⑂ ● No Nested Session
                                 │    – 14s ago – …
                                 │
                                 └─⑂ ● Has A Nested Fork
                                      – 15s ago – …

Why

The two modes have different problems to solve. Compact packs its rows with
nothing between them, so depth alone already carries the hierarchy — the rails
are ink in the view whose entire purpose is density. Full mode breaks each
session into a title row, a detail row, and a spacing row; without a
continuous rail a child stops reading as attached to its parent across that
gap, which is exactly what the rails were introduced for in #1156.

Archived-children rows follow the same rule, so nothing in the list draws a
rail compact mode doesn't.

Screenshots

Compact, main on top and this branch below — same fleet, same width. The
⑂ marks, the ▼ disclosure, and every title stay in their columns; only
the rails go.

Full mode on this branch, unchanged — rails still run through the detail and
spacing rows:

Compact, full frame

Tests

• New test asserts the compact prefix is blank at every depth and the same
  display width as the full-mode prefix it replaces, so dropping the rails can
  never shift a row.
• The existing rail test now pins full mode explicitly.
• cargo test --workspace green.

Relevant binary: construct (this PR only touches crates/cli).