feat: add dsl::toAscii, a console listing of a parsed graph

[psyinf]

Implements the first step of TODO item "dsl::toAscii: console rendering". Stacked on #18 (which is stacked on #17). Merge those first; this PR then retargets to main.

What

dsl::toAscii(program, style = AsciiStyle::plain) prints a parsed graph as an indented listing and needs no external tool:

in
`-> Parse -> msg
    +-> Validate -> valid
    |   `-> Summarize (merge, see below)
    `-> Summarize (merge, see below)

(msg, valid)
`-> Summarize -> out.stats

• One tree per graph input (in, in.<key>). Each line is a stage in DSL syntax (Print(prefix="[tap] ")) followed by the edge it produces, or end.
• Fan-out shows as siblings.
• A merge appears as a reference in the trees of its inputs. Once all of its inputs have been shown, it gets a tree of its own, headed by its group (with slot names if the group names them: (name: proper, greeting: shouted)).
• AsciiStyle::unicode draws with ├─► └─► │ ─► (UTF-8, written as byte escapes because the build has no /utf-8). A Windows console needs chcp 65001 to show them, which is why plain is the default.
• An invalid program (a cycle, a missing producer) still prints every stage exactly once and terminates; a repeated stage shows as (see above).

Not in this PR

The 2D box layout (layered columns, crossing reduction, edge routing) stays in TODO.md as item 7, now scoped to just that, to be done only if the listing turns out not to be enough.

Docs / tests

README ("Visualizing" + feature list), EXAMPLE.md section 6 (listing of the section 3 graph), CHANGELOG. Four new cases compare the full output text: merge/fan-out, arguments/dead ends/both styles, invalid program, named group. All 114 tests pass (MSVC debug, ASan).