[Feature] Type-assertion debt: the ~197 remaining as-unknown-as / as-never sites

[pathosDev]

Size / Priority

• Size: M — mechanical but broad, and gated on tooling.
• Split out of [Feature] Type-assertion debt audit (as unknown as T / as any) #256.

Rationale

#256 asked for a type-assertion debt audit. Its measurable half is done: there are 0 as any casts left in src/.

What remains is the wider assertion debt that issue also swept up, and it is a different and larger job than removing as any:

• 141 as unknown as sites
• 56 as never sites
• 33 any type positions

Rather than keep #256 open indefinitely against a moving target, the finished half was closed and the remainder tracked here.

Notes

• Some of these are legitimate and will stay — the variance-breaking casts at framework seams (e.g. Props.create boundaries, testkit probe refs). The deliverable is a catalog separating those from the ones that can go, not a blanket count of zero.
• A lint rule to hold the line is not possible today: the repo has no linter. [Feature] Adopt Biome as the repo formatter + linter #417 (adopt Biome) is the prerequisite.
• Related: [Feature] Topic<T> wrapper for type-safe pub-sub (vs as never casts) #273 covers the as never casts specifically in pub-sub, and [Feature] Tighten CRDT-codec type bounds: Crdt<unknown> not Crdt<any> #272 the Crdt<any> bounds.

Acceptance criteria

• A catalog of the remaining assertion sites, split into "justified at a seam" and "removable".
• The removable ones removed.
• Once [Feature] Adopt Biome as the repo formatter + linter #417 lands, a rule that prevents new unjustified ones.