docs(model): reconcile 0007 Series with the shipped step-1 reality

The `series` library extension shipped (35a9bc6); model 0007's
Series section was an aspirational sketch with systematic defects
that no longer match the live tool. Bring it to present-state per
the honesty-rule:

- STATUS: the library-extension milestone now reads step-1 shipped
  (primitive Series over {Int, Float}, Bool deferred), with SoA
  step-2 (#62) still pending. INDEX.md row updated to match.
- The `series` code listing is replaced with the shipped source
  (crates/ailang-kernel/src/series/source.ail, doc-strings elided),
  cited as the source of truth. Every defect the sketch carried is
  corrected in the shipped form it now shows: param-in (a Int Float)
  not (... Bool); `(new RawBuf lookback)` with no type-arg; bare `a`
  for the element type variable (not `(con a)`); forall-bound op
  signatures; `lt` not `<`; and the at-index formula with the
  correct sign on i — which already matched the section's own prose
  (`head - 1 - i + 2*lookback`), so the sketch was internally
  inconsistent and the prose was right.
- The SMA worked example uses `ge` (not the non-existent `>=`) and
  emits each average followed by a newline, matching the committed
  pin fixture so the doc and the test agree.
- The "four constructs not in today's AILang" framing is now
  present-state: the mechanisms shipped via raw_buf, Series itself
  via the step-1 library extension.

Both edited module blocks parse against the live tool.

refs #61

design/INDEX.md

@@ -110,4 +110,4 @@ is the default.
 | authoring-surface | onboarding / evolves | design/models/0001-authoring-surface.md |
 | prose-projection  | onboarding / evolves | design/models/0006-prose-projection.md |
 | pipeline          | onboarding / evolves | design/models/0003-pipeline.md |
-| kernel-extensions | onboarding / evolves (mechanisms milestone closed 2026-05-28; ratified end-to-end by the `ailang-kernel/src/raw_buf` base extension; series milestone pending) | design/models/0007-kernel-extensions.md |
+| kernel-extensions | onboarding / evolves (mechanisms milestone closed 2026-05-28; ratified end-to-end by the `ailang-kernel/src/raw_buf` base extension; series library extension step-1 shipped via `ailang-kernel/src/series`, SoA step-2 #62 pending) | design/models/0007-kernel-extensions.md |