Implement bounded v2 explode, reroll, and keep-and-explode options

## Parent

- #7

## What to build

Implement accepted validator-driven, budgeted v2 generation options on the shared RFC-0002 context and RFC-0003 generation trace primitives.

## Frozen contract

- `rV` redraws each matching numeric or text source node once and replaces it.
- `RV` redraws and replaces while each new value matches.
- `aV` redraws matching numeric nodes once and adds the contribution.
- `eV` recursively draws and adds while the newest numeric draw matches.
- `KN` / `KlN` atomically expand to maximum-face explode followed by keep-high / keep-low.
- `!`, bare `e`, `rN`, `roN`, `RN`, and `aN` are compatibility forms only.
- Generated, replaced, retained, and discarded nodes preserve RFC-0003 parent/cause identity.

## Acceptance criteria

- [ ] Every form matches RFC-0001 option and migration case IDs.
- [ ] All conditions use the shared Validator contract.
- [ ] Option order preserves every generated, retained, replaced, and discarded node and trace operation.
- [ ] Every draw consumes request-wide RNG, generated-value, collection, trace, and work budgets.
- [ ] Impossible reroll-until and explosion conditions terminate with structured replayable resource errors.
- [ ] Seeded Rust and installed Python payloads agree and validate against RFC-0003.

## Blocked by

- #7
- #18
- #27
- #40
- #41

Start by reviewing the shared RFC-0002 context and RFC-0003 generation trace primitives, then inspect the blocked work in #7, #18, #27, #40, and #41. Done means every listed option and migration case is validator-driven, budgeted, trace-preserving, and produces matching seeded Rust and installed Python payloads validated against RFC-0003.

Written by the indexing model from the issue text.