feat(maimu): move Maimu out of core into an installable plugin

[gbradham]

Last of three. Stacked on #136 (ble_address field), which is stacked on #135 (node extension points). Retarget to main as those land.

One lab's stimulator does not belong in every install. The device and node are unchanged; what changes is that they ship as glider-maimu alongside glider-harp rather than being bundled.

Moving it was the point

#135 shipped the plugin node extension points with zero consumers. Making Maimu the first one is the only way to find out whether they were good enough — and they weren't, in two ways. Both gaps are general, and both are fixed here:

A plugin node could not declare its canvas ports. setup_node_ports fell back to one generic exec-in/exec-out for any type not in its hand-written table, silently drawing the wrong shape for anything else. That is the same fallback that left Behavior Input's On Enter unreachable in #132. Ports now come from the node's own NodeDefinition, which every node already declares — so this also fixes every built-in node missing from that table, Device Action included.

A plugin node could not be bound to a device. The properties panel offered a device selector by matching a hardcoded list of type names, which no plugin can join. A node now declares REQUIRES_DEVICE, set on HardwareNode, so anything needing hardware gets the selector.

What survives the move

• Scan. The address field becomes the ble_address schema type from feat(gui): a ble_address schema field, so any BLE device can offer Scan #136. As a plugin it cannot be special-cased by name in the hardware panel, which was the only way a device got Scan before.
• The closed loop. test_behavior_triggers_stimulus.py moves to the plugin and still drives all four hops, asserting on the bytes reaching the peripheral. That is where it belongs: core supplies the behavior trigger, the plugin supplies the stimulus.
• Everything about the protocol — off-before-disconnect, pulse validation, name-vs-address portability, no read action.

What does not

The node's Mode / Period / Duration move from a hardcoded editor branch to a declared PROPERTIES_SCHEMA. Period and Duration no longer grey out outside Pulse mode — the schema form has no notion of one field depending on another. The cue moved into their help text. Adding conditional fields to the schema form would be its own change.

Core tests that mentioned Maimu

Four used it as a convenient BLE device. They now use the core BLE types, because what they test — driver rebuilding on open, the name settings collision, serialization round-trips — was never about Maimu. One is strengthened in passing: the port test now uses Device Action, which has a genuinely interesting shape (3 in, 2 out) rather than Maimu's 1-and-1.

Wiring

CI installs the plugin editable and runs plugins/glider-maimu/tests/ with coverage, matching how glider-harp is handled. Docs point at the plugin instead of listing a built-in.

Verification

• Core: 3607 passed, 4 skipped
• Both plugins: 498 passed
• ruff and black clean across src/, tests/ and plugins/