fix(relay): enforce scoped budgets consistently at every ingress

[albertovincenzi]

Problem

Scoped-budget validation disagreed across the two supported ingress paths. HTTP rejected a missing scope even when whenOp made that budget irrelevant to the item. A producer writing directly to a user-owned ingress queue had the opposite and more dangerous behavior: when an applicable scope was absent, the relay silently skipped that counter and forwarded the item under only the other budgets.

Fix

• add one pure missing-scope resolver that applies whenOp before requiring a scoped key
• use it at the HTTP door, so unrelated operations are no longer rejected
• use it in relay classification, so malformed direct-ingress items enter the existing poison and dead-letter path rather than bypassing a limit
• classify foreign shared-queue messages first, preserving the rule that one path never validates another path payload
• add core coverage for applicable versus unrelated operations and relay coverage for direct-ingress poison classification

Dependency

This PR is intentionally based on #10, which first makes shed-mode budget selection match relay selection. The new helper completes that same applicability contract for scope validation.

Review notes for Alice

The relay does not drop a malformed item silently: it uses the existing poison path, which nacks the head with a reason and increments dead-letter telemetry. A missing scope is immutable in the queued payload and cannot become admissible on retry, while forwarding it would violate the declared per-key limit. Please pay particular attention to ordering in classify: foreign ownership is resolved before payload validation.

Verification

• cargo test -p gate-core
• cargo test -p gate-server --lib
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

[alice-viola]

Landed on master via #67 (merge commit 944ee9b) as part of the 62-PR integration — this PR's head commit 3a0a443 is an ancestor of master. GitHub could not mark it merged automatically because its base is fix/shed-applicable-binding-budget, not master. Closing as landed.