feat: type-checked merge inputs

[psyinf]

Stack (bottom → top)

1. feat: add GraphContext, a graph-scoped side channel for stages #9 — GraphContext
2. feat: type-checked merge inputs #10 — type-checked merge inputs (this PR)
3. feat: add finish(), the end-of-stream lifecycle hook #11 — finish() lifecycle hook

Summary

A merge stage may now declare the type of each of its slots, so the DSL checks the edges of its fan-in group when the graph is built instead of failing with a std::bad_any_cast on the first message. This closes the only edges the library did not check: GraphPlan checks every single-input edge, but a merge only declared MergeInputs = std::vector<std::any>, so (msg, valid) and (valid, msg) both validated and ran — silently producing a different result when the slots share a type.

What's new

• MergeStage / MergeSlotTypes (new MergeStage.hpp) — how a merge declares its slot types. MergeInputs moved here from MergeFilter.hpp, which still provides it.
• AnyMessageFilter::mergeInputTypes() — exposes them to the DSL. The adapter picks them up from any stage implementing MergeStage, so MessageFilter itself is unchanged.
• TypedMergeFilter<Out, Ins...> — fixed arity, one type per slot. merge() receives the slots as std::optionals (std::nullopt = a hole left by a dropped path), so user code needs no any_cast.
• UniformMergeFilter<In, Out> — any number of slots, all of one type: N variants of the same computation.
• registerTypedMergeFilter<Out, Ins...>(name, fn) — the same from a lambda, the typed counterpart of registerMergeFilter.

New diagnostics, in the usual line:column style:

3:20: slot 1 of 'Report' expects 'class std::basic_string<char,...>' but edge 'length' carries 'unsigned __int64'
5:9: stage 'MergeLabel' takes 2 inputs but the group has 3

Compatibility

Additive. MergeFilter / registerMergeFilter declare no slot types, so their edges stay unchecked and existing graphs behave exactly as before.

Notes

• A hand-written merge stage (MessageFilter<MergeInputs, Out> registered via FilterRegistrar) can also implement MergeStage to be checked.
• registerMergeFilter's combiner is copied into every instance, so captured state is shared between instances. That is now documented at the registration function; per-instance state is what the class-based merges above are for.

Testing

New MergeTests (9 cases): typed slots and holes, the lambda registration, uniform merges of 2 and 3 slots, a mis-wired group and a wrong slot count reported at build time, an untyped merge still accepted unchecked, per-instance state, and direct use outside a graph. The sample app gains a typed merge (Report) whose real output and diagnostics are what EXAMPLE.md shows. Full suite: 77/77 locally (MSVC debug, ASan).