fix: reject unstampable payloads before shared routing

[albertovincenzi]

Problem

Gate uses the reserved _gate.path field to decide which path group owns a frame on a shared interior queue. JSON scalars and arrays cannot carry that field. The relay nevertheless forwarded them, so the compiler-selected unstamped owner could charge and route every copy as the wrong path after a convergence.

This is data-plane corruption rather than a presentation issue: the payload may leave through the wrong continuation and its traffic is attributed to the wrong path.

Fix

• Mark only destinations whose queue has multiple path readers as requiring a path stamp.
• Dead-letter an unstampable payload at the last unambiguous upstream stage before such a destination.
• Preserve scalar and array payloads on linear routes and terminal pushes, where path ownership is not needed.
• Expose requiresPathStamp in the topology response so the compiled constraint is inspectable.

Verification

• Compiler test distinguishes a shared convergence destination from an unrelated fan-out branch and from a linear hop.
• Relay unit test covers rejection, object acceptance, and scalar compatibility on a non-shared route.
• Broker-backed live test puts a scalar ahead of valid work and verifies that only the scalar is dead-lettered and the valid object crosses exactly once.
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check

Review / dependency

This PR is intentionally stacked on #25 because it extends the message-classification path introduced there. Review commit 6994cd2 relative to fix/enforce-applicable-scoped-budgets. Merge order: #10, then #25, then this PR.

[alice-viola]

Landed on master via #67 (merge commit 944ee9b) as part of the 62-PR integration — this PR's head commit 6994cd2 is an ancestor of master. GitHub could not mark it merged automatically because its base is fix/enforce-applicable-scoped-budgets, not master. Closing as landed.