Publish collector health on the bus as schema v5 #8

SignalName::CollectorUp at discriminant 11, SCHEMA_VERSION = 5. Value 1.0 up, 0.0 down. Untagged; published once per Source by the daemon's supervisor.

The hub keys its last-value cache by (name, tag) (hub.rs:80), so two signals differing only by source overwrite each other. That is latent today and blocks one health variant per source, so the key becomes (name, source, tag).

Done when:

• CollectorUp exists at 11, SCHEMA_VERSION is 5 and documented as the 1.0 contract.
• The hub and terminal-garden key by name, source and tag; a test asserts two sources sharing a name coexist.
• Swift SCHEMA_VERSION, both canonical frames and the macos-collector byte table move to v5.