Reorganize sampling kernels into clear code paths with shared utils

[oliverdutton]

Restructure tallax/vllm/ from the fix-topp-integer branch into three
clear packages:

vllm/utils/ - Shared utilities used by both code paths:

• binary_search.py: Monotonic f32<->u32 conversion, binary search
• high_precision_uint.py: U48 48-bit arithmetic, modulo_u128_u64, RNG

vllm/fullvocab/ - Full-vocabulary binary search path (never sorts):

• topk_mask.py: Binary search in f32 space + stable boundary detection
• topp_mask.py: Binary search in i32 probability space with U48 sums
• kernel.py: Combined topk+topp+sample Pallas kernel with debug_results

vllm/reducedk/ - Sort-based reduced-k path (bitonic sort to k, then operate):

• top_p_and_sample.py: top_p_integer_mask on sorted subset, re-sort to
  original order, sample via modulo_u128_u64

vllm/reference.py - Pure JAX reference (no Pallas):

• Uses jax.enable_x64 for exact i64 arithmetic
• Python arbitrary-precision u128 % u64 via pure_callback
• Ground truth for both kernel paths

Debug support: fullvocab kernel accepts debug=True to return a nested dict
of int32[1] SMEM values for verifying intermediate stages match reference.

Also:

• Cherry-pick stable dynamic_topk from stable_topk branch (stable sort in
  divide-and-filter, >= swap for stable index ordering, reverse loop order)
• Fix syntax errors in cherry-picked code (missing parens)
• Add import operator for stable comparison
• Add sparse_random_bits() extracted from sparse_random_uniform()
• Add map_reduce() to tax/utils.py for parallel chunked reduction
• Add comprehensive component tests (binary_search, U48, modulo, topk_mask,
  topp_mask, reference greedy, fullvocab vs reference end-to-end)

https://claude.ai/code/session_01Rd5Nd2h4ZSmMW5wAAkiuwt