Split legacy Pre-Planning: fresh Target Selection page (Virtual Fit follows)

[peterhollender]

Continuing the split-session refactor (SlicerOpenLIFU#631, SESSION_SPLIT_DESIGN.md). The legacy Pre-Planning page in pages_legacy/preplanning_page.py is ~2100 lines and mixes two distinct workflows that share little UI state.

Why split

The legacy page has two tables and two "current target" concepts:

• The Targets table with its own selection + edit mode.
• The Virtual Fit section's algorithm-input target combo, which drives a different "current target" for VF display.

These can point at different targets simultaneously, producing an ambiguous 3D scene ("which target's VF handles am I looking at?"). The legacy code accumulated a lot of ad-hoc re-entrancy guards (_input_update_in_progress, _target_table_edit_in_progress, _vf_interaction_in_progress, _placement_in_progress) to keep the two halves from stepping on each other during cross-page cascades.

Splitting into two pages puts each workflow in a self-contained page with a single canonical "current target":

• Target Selection page — targets table + Add / Import / Edit / Remove; the loaded PlanningSession's targets are the only thing this page cares about. 3D shows the volume + all target fiducials. No transducer, no skin, no VF transforms.
• Virtual Fit page (follow-up commit) — target picker at top + VF results table + Auto-Fit / Manual / Edit / Approve / Remove. 3D shows the volume + the currently-picked target + skin mesh + transducer at the selected VF pose.

Future: a Volume Segmentation page slots in before Target Selection.

This issue's scope

Land the Target Selection page first. The Virtual Fit page is a separate follow-up.

Target Selection page

New file OpenLIFU/OpenLIFUApp/pages/target_selection_page.py.

• Widget (OpenLIFUTargetSelectionWidget):
  • Header + subject / volume / protocol context labels.
  • Targets table: Color swatch | Name | ID (hidden by default) | R | A | S | Show | Jump.
  • Action row: Add Target | Import… | Edit (toggle) | Remove.
• Logic (OpenLIFUTargetSelectionLogic): direct callable API for add / import / rename / move / remove / toggle-visibility. Every mutation calls mark_session_dirty -- the openlifu Session lives in memory until Save.
• Dialog (ImportTargetDialog): pick an existing scene fiducial or load a .mrk.json / .fcsv from disk. Single call site so stays in-file per rule 7.

Navigation

• Planning Session Overview grows an Edit Targets… button that navigates to Target Selection.
• Target Selection uses the host's footer Back-to-Home for exit -- no per-page navigation button.
• Timeline participation is deferred until we have a matching Virtual Fit page.

Cross-page cascades

The legacy code cascaded target-remove / target-move into VF-approval revoke, TT-approval revoke, and solution-delete. For now:

• Removing a target from Target Selection removes the fiducial from the scene AND removes any virtual_fit_results entries for that target from the openlifu session dict (in memory only).
• Moving a target unsets the target's approval flag in virtual_fit_results (in memory only).
• TT / solution cascades will land with the TT and Solution Generator pages -- for now those live only on the legacy pages, which are still in pages_legacy/ for reference.

Docs

• New docs/pages/target-selection.md with the standard layout / API / signal-flow / acceptance-tests shape.
• docs/pages/README.md -- add entry.
• docs/architecture.md -- update the page-navigation diagram: Overview → Target Selection.
• SESSION_SPLIT_DESIGN.md section 5.4 -- update the Planning workflow to reflect the split (Overview → Target Selection → Virtual Fit → Solution Generator → Overview).

Non-goals

• No Virtual Fit page in this commit.
• No Volume Segmentation page in this commit (comes later).
• No timeline registration yet -- Target Selection + Virtual Fit + Solution Generator will get put on the timeline together once all three exist.
• No changes to the legacy code under pages_legacy/; it stays for reference.

Acceptance

1. Load a PlanningSession. Planning Session Overview appears. Verify it now shows an Edit Targets… action.
2. Click Edit Targets… → Target Selection page opens. Targets table is populated from the loaded session.
3. Click Add Target. Click in a slice view. Fiducial appears, targets table adds a row, session marked dirty.
4. Click Import…. Pick an existing scene fiducial. Row appears; session dirty.
5. Click Edit. Change a target's Name and R/A/S cells. Fiducial updates. Session dirty.
6. Click Remove on a target. Fiducial removed. If the session had virtual_fit_results for that target, they're cleared from the in-memory dict.
7. Toggle a target's Show checkbox. Fiducial visibility flips.
8. Click Jump on a target. Slice views snap to the target.
9. Save via toolbar → session written to disk. Data Manager shows the updated target count.
10. Discard on Exit → in-memory changes drop; disk unchanged.

Relates to SlicerOpenLIFU#631, SlicerOpenLIFU#633, SlicerOpenLIFU#636.