docs: add TODO.md, the tracked list of planned work

[psyinf]

Collects the open extension ideas in one file, so planned work has a single
place to live and PRs can refer to an item instead of restating the design.

TODO.md lists, in overview and then one section each:

1. More examples for the newer features — GraphContext, finish(), a
   stateful merge, JoinFilter, a nested graph, a Void sink, the in-band tick
   pattern. The follow-up PR to this one.
2. Named merge slots — (raw: msg, checked: valid) -> Summarize.
3. Several named graph inputs — in.orders, in.quotes, mirroring
   GraphOutputs.
4. Stage labels and typed access — Summarize@stats, graph.stage<T>("stats").
5. Track which stage short-circuited — moved here from the README roadmap.
6. Config-aware registerMergeFilter — a fresh combiner per instance.
7. Injectable registry, duplicate detection — instead of a singleton that
   silently overwrites.
8. Documentation — 0..n outputs, fan-out copies, stateful stages.
9. Known limitations to lift — nested stage arguments in the DSL, type
   checking inside GraphOutputs.

Each item states what the library does today, the design gap, a proposal with an
API sketch, its compatibility impact and the workaround available in the
meantime. Nothing in it is implemented; the two items of the original list
that have shipped (type-checked merge inputs, finish()) are named as done and
left to the README.

The README roadmap section now summarizes the list and links to it.

Docs only: no library, app or test code changes.