Re-scope the public-API snapshots

[blooop]

Question

The frozen api module is ~37 of 2,234 snapshot lines; 1,400+ binary-surface pub items (including ~55 allow(dead_code) ones nothing constructs) are pinned exactly as hard as the promise, so the snapshot diff has poor signal. Meanwhile pub use devlaunch_runner::* renders as one unexpanded glob (public-api.txt:2214), so adding a Runner method or an Outcome arm — breaking for any external implementor — changes nothing, and devlaunch-runner has no snapshot of its own.

Decide: a separate snapshot for devlaunch-runner; whether and how to shrink core's binary surface (facade module, pub(crate) + targeted exports) or split snapshots api-vs-rest; the fate of the dead pub items. Then slice the builds (see the map's Not yet specified).

[blooop]

human-decided: A runner snapshot plus an api-vs-rest split of core's snapshot; no visibility shrink, as asked and as answered.

devlaunch-runner gets its own checked-in cargo public-api snapshot: its whole trait surface — the one an external Runner implementor actually sees — currently enters core's snapshot as a single unexpanded glob line (public-api.txt:2301) with no snapshot of its own, so a breaking Runner/Outcome change moves nothing. Core's snapshot splits into an api-only file (the frozen wf promise, 37 lines today — a diff there is a breaking change by definition) and a rest file (a routine tripwire over the binary surface, regenerated freely). That restores the diff's signal without touching visibility.

Shrinking the binary surface was ruled out on evidence: dl names flows::/domain::/clients:: directly in 9 files, so demotion breaks dl, and facading it is churn with no consumer today (nothing in-repo uses api; wf still consumes dl as a subprocess). The review's "~55 allow(dead_code) pub items" softened under measurement: nearly all are pub(crate) or cfg_attr(not(test)); the 2 bare-pub ones are tidied in the build.

Graduates one build (the snapshot re-scope), which inherits the blocking edge #314 and #316 held against this ticket, so the snapshot files move once.