Implement v2 keep, filter, sort, count, and unique option semantics

## Parent

- #7

## What to build

Implement the accepted non-random selection/query pipeline on the shared typed Value and RFC-0003 trace graph.

## Frozen contract

- `kN` / `klN`: stable keep-high / keep-low.
- `dN` / `dlN`: stable drop-high / drop-low.
- `fV`: retain items matching an each-scope Validator.
- `s` / `sl`: stable descending / ascending sort.
- `cV`: return Scalar count while retaining matching source-node provenance.
- Postfix `u`: retain first occurrences without refill.
- Counts are positive and bounded; wrong kinds and oversized keep/drop requests fail before evaluation.

## Acceptance criteria

- [ ] Every option matches RFC-0001 case IDs and RFC-0003 Value/Trace shapes.
- [ ] Filter and count use the shared Validator evaluator.
- [ ] Zero, oversized, wrong-kind, and incompatible parameters return spanned validation errors.
- [ ] Options execute left to right; selected and discarded RollNodes remain reachable.
- [ ] V1 `kh`, `kl`, `dh`, `dl`, and ascending `s` have executable migration cases.
- [ ] Rust, installed Python, CLI, and documentation agree for representative compositions.

## Blocked by

- #7
- #21
- #39
- #41

Start by reading RFC-0001 and RFC-0003, then inspect the work and dependencies in issues #7, #21, #39, and #41. Use the frozen contract and acceptance criteria as the completion checklist, including representative Rust, installed Python, CLI, and documentation compositions.

Written by the indexing model from the issue text.