Raise assert density in the three thinnest library modules, enforcing invariants that already hold. Asserts only, no behavior change.

update.zig: guard the checkout-state transition. applyCheckin asserts the desired managed set equals the target plan membership (validateTopology proves the paths unique); requireClean asserts a clean tree's plan membership equals the recorded managed set; recordUndoImage asserts the affected union covers both operands; commitMaterialization self-guards that the committed record names exactly the staged check-in and that the managed set is its membership; the merge path asserts one rollback pre-image per path and a shared pre-merge managed baseline.

serve.zig: a servable request is a POST bearing a request-valid compressed media type (§4 L759-764); an accepted body honors the §4 L1148-1149 ceiling; a compressed reply is application/x-fossil (§4.1); the accept loop honors its request bound.

transport.zig: a framed container carries its 4-byte length prefix (§4.1); an unframed reply stays within reply_body_cap (§4 r9); a followed redirect lands only on an http/https transport (§4 r12/r13). closed check-in: c9fb87f554 user: dmestas tags: h5a-asserts