feat(plugins): allow bounded concurrent BLE bindings

[MaxRink]

Summary

Raise the plugin host's per-generation BLE binding cap from one to four so one installed driver can own concurrent physical scale bindings. The multi-scale requirement is three simultaneous bindings—one primary and two auxiliary scales—while each device keeps its own claim, session fencing, permission checks, and teardown reservation.

A fifth binding is rejected and explicit limits remain bounded from one through four. This changes the host resource policy only; it does not add a connection manager, protocol abstraction, or public REST endpoint.

Linked Issue

Related #851. Background: closed #809; first-stage driver #846 and current multi-scale consumer #859.

Verification

• Combined Linux Flutter gate for the current publication stack: 4,315 passed, 1 skipped, 0 failed; analyzer clean (48.0s). Source was unchanged during the gate.

• Combined simulated runtime checks passed for the current stack; no production fixes were needed during runtime validation.

Impact

Existing one-binding drivers remain compatible. Drivers that opt into the higher bound can keep independent primary and auxiliary device sessions without duplicating the installed plugin.

Contributor Responsibility

AI-assisted development is allowed. The submitter remains responsible for the submitted work.

• I have reviewed and understand all changes in this PR and take responsibility for their correctness, security, behavior, licensing, and provenance, including any AI-assisted or AI-generated work.

[tadelv]

Maintainer direction for 0.9.0: this is P0 generic driver-runtime v2 work. #851 is accepted with the proposed default/cap of four active BLE bindings per plugin generation; see also the execution tracker #890.

Please keep this PR strictly to host resource policy and per-binding isolation. Do not add Scale roles/workflow semantics here. #858 and the 0.8.7 auxiliary core #862 have now landed, so update/rebase against current main as needed and keep the binding tests independent of those consumers.