feat(tui): add recursive session grouping tree

[jlongster]

Summary

• Add a generic recursive grouping tree driven by per-entry grouping paths.
• Support ordered associative seam merges and depth-first splits without flattening.
• Cache total leaf counts on groups so wrappers do not affect future transcript budgets.
• Preserve input trees and untouched node identities across seam operations.

This is the pure data-structure increment following #48393. It is not wired into production rows or rendering yet, so current TUI behavior is unchanged. Message normalization, replay deduplication, presentation identity and the production rules adapter remain follow-up work.

Tests

• Focused grouping suite: 18 passed, 48,097 assertions
  • nested/two-phase grouping
  • recursive compatible and incompatible seams
  • untouched identity and input immutability
  • every depth-first split boundary
  • all two-way and three-way partitions
  • exhaustive four-entry sequences over six representative paths
  • deeper-than-two-level paths and projection-layer duplicate ownership
• bun typecheck from packages/tui: passed
• Full TUI suite: 1370 passed, 4 skipped, 0 failed, 155,952 assertions across 144 files
• git diff --cached --check: passed

Based on merged origin/v2 at 0c1dfa9186.