talky/splitter: generic sidecar cut, one message per section on route `sidecar`

[mmeyerlein]

Part of #604. The splitter cuts a sidecar block (legacy memory still accepted and mapped to section memory), parses ONE object, and emits one message per top-level key on route sidecar with hop.section=<key>; body {"messages":[],"section":"<key>","payload":<obj>}. Unknown or non-object sections are dropped with a hop note; a block that cannot be parsed is cut and recorded as hop.sidecar=malformed with nothing emitted (GH #534 stands). The extraction port of the composite becomes sidecar. Tests gh379/gh534 follow. Talky bump (minor).

[mmeyerlein]

talky@5.1.0: the splitter cuts one sidecar block and emits one message per section on route `sidecar` with `hop.section`; legacy memory maps to section memory; malformed blocks are cut and emit nothing. Built on the sidecar wave, integration gate GATE-SUMMARY integration 31519ad 3/3 296s GREEN (all other stations green on the first run, one prose fix in between). Local commit 31519adc; public with the next export.