Credence.Syntax.FixDoBlockFusion (credence v0.8.0)

Fixes LLM confusions between the do ... end block form and the , do: one-liner form of definitions.

Diffusion code models (Dream, DiffuCoder) emit these fusions constantly - they are the single most common reason a draft fails to parse.

Detected patterns

def f(x), do        - comma followed by a block opener at end of line
def f(x), do expr   - one-liner missing the colon (midline)
def f(x) do do      - doubled block opener
def f(x) do: expr   - block opener fused with one-liner syntax
def f(x), do: expr end - one-liner with a stray trailing `end`

Bad

def push(stack, value), do
  [value | stack]
end

def double(number) do: number * 2 end

Good

def push(stack, value) do
  [value | stack]
end

def double(number), do: number * 2