fix(blocks): Dict block exposes N inputs and returns list

[PhotonicVelocity]

Summary

Two bugs in bdsim.blocks.connections.Dict. Together they make
bd.DICT([...]) unusable for any keys list longer than 1 (and broken
even then, since the output isn't list-wrapped).

Bug 1 — nin not propagated

class Dict(FunctionBlock):
    nin = 1  # ← class attribute
    def __init__(self, keys: list[str], **blockargs: Any) -> None:
        super().__init__(**blockargs)   # ← nin=len(keys) missing

bd.DICT(["a", "b", "c"]) exposes only input 0. Wiring to
dict_blk[1] or dict_blk[2] fails at compile with:

error connecting wire ... --> .../dict.0[1]: list index out of range

Bug 2 — output() returns bare dict

def output(self, t: float, inputs: list[Any], x: Any) -> Any:
    return {key: inputs[i] for i, key in enumerate(self.keys)}

bdsim expects output() to return list[Any] (one entry per output
port). Even after fixing bug 1, running a diagram with a Dict block
fails:

AssertionError: block Block.function(... type=dict, nout=1) output
... must be a list: <class 'dict'>

Fix

nin: int = -1
...
super().__init__(nin=len(keys), **blockargs)
...
def output(self, t: float, inputs: list[Any], x: Any) -> list[Any]:
    return [{key: inputs[i] for i, key in enumerate(self.keys)}]

Mirrors the variable-input pattern in Mux (~30 lines below in the
same file), which is correct: nin: int = -1 + super().__init__( nin=nin, ...) + list-wrapped output.

History

The original class Dict introduced in 5b2bc33 (May 2021) was
actually an Item-extractor (1 dict input → 1 value out). A later
refactor (f7ba8b4) split Item into its own class and rewrote
Dict.output() to do the correct N→dict pack, but kept the
nin = 1 from the original Item-as-Dict implementation and forgot
the list-wrap.

Base

This PR is against 12b26c2 ("add the tagline") rather than current
main HEAD because of #34 (current main can't import bdsim — a
missing graphics_block.py from a recent move). The fix is in
connections.py and is orthogonal to the graphics breakage, so it
should rebase cleanly onto fixed-main.

Test plan

• bd.DICT(["a", "b", "c"]) reports nin == 3.
• Wire three CONSTANT blocks to the three slots; diagram compiles
  and runs.
• Output dict has the three expected keys with the wired values.

No tests added to the bdsim test suite — leaving that to a broader
testing pass.

[codacy-production]

Not up to standards ⛔

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