tui: reserve the session row's disclosure and lineage gutter

[edwin-zvs]

Problem

The hierarchy pass (#1156) appends the children disclosure (▼/▶) and the
fork lineage mark (⑂) as extra cells after the tree rails. So two
sessions at the same depth no longer share a left edge: the moment a session
grows a subagent or a fork, its own title jumps one column right of its
childless siblings'.

Before #1156 the indent helper subtracted a cell whenever the disclosure was
present, which is why a project's members lined up whether or not they had
nested sessions. That compensation was dropped when the indent became tree
rails.

Fix

Give every row hanging off the tree one shared marker column, one step
past identical rails, reserved whether or not it is used:

row	marker
session with children	▼ / ▶ disclosure
fork	⑂ lineage mark
archived-children row	▸ / ▾ disclosure
plain session	space

▼ construct-dev  (3)
├─  ● No Nested Session          <- no nested sessions
├─▼ ● Has A Nested Fork          <- has nested sessions; same left edge
├─⑂ ● A Fork                     <- lineage in the same column
└─▸ 2 archived                   <- as the archive disclosure already was

So nesting state changes the glyph, never the geometry; a child's status
glyph lands under its parent's title again; and a fork row and an archived
row differ by their glyph rather than by their offset.

Where a session could claim the cell twice, the disclosure wins — it's the
affordance the operator can act on. No row is both today: the list gives forks
no disclosure, always rendering their descendants expanded. If that changes,
lineage needs a new home rather than a second reserved column, which the spec
and a code comment both now say.

Net width is unchanged from main. The disclosure's click column still
follows the rails, so hit-testing needed no update.

Screenshots

Top row is main, bottom is this branch — same fleet, same terminal size.
In the "before" half Prepare Patch Release's status dot sits one column left
of Session Statuses'; in the "after" half they align.

Full frames

before:

after:

The demo uses forks at depth 0/1/2 because groups can't be created from the
CLI, but a project's members hit the identical shift — they're depth-1 rows
whose only difference is the reserved cell.

Tests

• Every marker is pinned to one cell across both fork/non-fork and all four
  has-children/expanded combinations, glyphs and precedence included.
• A second test pins that a fork row and an archived-children row under the
  same parent carry the same rails and the same marker width — i.e. they share
  the column rather than each choosing an offset.
• cargo test --workspace green.

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