Create Stack Proposal

POST /projects/{id}/stack-proposals body — the canvas door (spec §5).

Composes (never subclasses) PlaceConditionSpec: the ledger stores the bare spec dump and the engine arms re-validate it with extra="forbid".

proposed → receipt carries the pending row (fresh propose also carries names/counts; an idempotent replay carries ONLY the receipt — names/counts are not stored on the row, and outcome describes the row's existence, not its current status). nothing_to_place → no row was written (spec §2) and reason drives the client copy: no_source_markups (A has nothing in scope) vs already_placed (every A is covered).