docs(pr): represent final state — drop roadmap-wave/phase framing from the changelog

Per review: the changelog should describe the final state, not the
development path. Removed the temporal 'Latest wave (Waves 1.3–6.1) —
added after the first changelog' umbrella section entirely and dissolved
its content into the proper topical sections, with all 'wave' terminology
and 'added after'/'previously absent'/'now reachable' path-language gone:

- np.evaluate folded into §2 (NpyExpr DSL): per-node result_type typing,
  fused reductions, out= rules, EXTERNAL_LOOP guard, measured speedups.
- out=/where=/dtype= ufunc kwargs folded into §5 as a parity subsection.
- WRITEMASKED/ARRAYMASK execution, VIRTUAL operands, and the size-1
  stride-0 / op_axes-OOB / write-broadcast / PARALLEL_SAFE / unit-axis
  fixes folded into §1 (capability matrix + bug list); masked-write
  corruption fix added to §10.
- buffer-pool window (1 B–64 MiB), pool-side GC pressure, finalizer
  suppression folded into §7; TL;DR memory bullet updated.
- canonical NpyIter benchmark, benchmark.yml CI, DocFX benchmark pages,
  and the honest frontier findings folded into §8/§15.
- 'NPYITER_GAPS_AND_ROADMAP … 6-wave plan' -> 'prioritized roadmap'.

Net: zero 'wave' occurrences; the 16-section topical structure is intact.
Same content (minus H1) pushed live to the PR #611 description.

Author: Nucs