feat(tui): add recursive grouping tree

[jlongster]

Base

Rebased onto v2 after #48393 merged. This PR contains only the grouping-engine increment.

Summary

Add the pure grouping engine before connecting it to the production session projection.

• Generic entries with configurable nesting paths, including activity -> exploration/reasoning/instructions.
• Ordered construction and associative recursive seam merges.
• Split by depth-first leaf count using cached subtree sizes; group headers count as zero.
• Readonly trees, immutable operations, and retained identity for untouched subtrees.

The current session projection and rendering are unchanged. This module has no runtime dependencies and is not wired in yet. The next integration step owns replay/overlapping-page deduplication and stable disclosure keys; seam merging explicitly concatenates disjoint ordered chunks.

Verification

• TUI bun typecheck: passed
• New module Prettier and staged whitespace checks: passed
• Local deterministic suite: 18 tests passed, 161215 assertions
• Exhaustively checked all 2801 sequences of up to four entries over seven grouping paths: every split and three-page partition, fresh/incremental equivalence, associativity, leaf counts and split/merge round trips
• Additional cases cover nested activity structure, native PartRef payloads, immutable inputs, unchanged subtree identity, invalid offsets, replacements/deletions and deeper nesting

The PR now includes six basic tests covering grouping, direct-child boundaries, recursive merging, splitting, empty chunks and invalid offsets (all passing; TUI typecheck also passes). The exhaustive diagnostic suite remains local at /tmp/opencode/grouping-tree.test.ts; the work log remains local too. No screenshot replay for this unwired pure module; production integration will repeat visual parity checks.