Add experimental opt-in direct webOS control for LG webOS TVs

[Staphylococcus]

Parent: #20

Phase: late 1.1.x migration rail.

Goal:
Let users and maintainers explicitly opt configured LG webOS TVs into direct webOS control once it has enough parity, while keeping bscpylgtv as the default production implementation for lg_webos during the initial migration window.

Scope:

• Add a durable TV platform field to the profile storage, for example tvs_primary_platform=lg_webos.
• If exposed through the current single-TV settings surface, map it to a semantic setting such as tv.platform while still storing it with the tvs_primary_* profile keys.
• Do not use the platform field to mean "which internal implementation is active".
• Add a clearly temporary implementation override for LG webOS direct-control opt-in only if needed. The exact name can be decided in implementation, but it should read like migration/debug plumbing rather than durable TV identity.
• Keep the default implementation for lg_webos as bscpylgtv.
• Wire the selected implementation through the same high-level TV policy surface.
• Log or print the active platform and implementation in diagnostic paths.
• Document direct webOS control as experimental and opt-in.

Acceptance criteria:

• Existing users see no behavior change unless they opt in.
• All current TV operations route through the implementation selected for the active LG webOS TV profile.
• WebOS failures are actionable and do not look like subprocess parse failures.
• Release smoke covers both default bscpylgtv and opt-in direct webOS modes.
• The config shape does not use tvs_primary_adapter=..., tv.adapter, or a global tv_backend=native style switch as the durable user-facing TV identity.

Out of scope:

• Making direct webOS control the default.
• Deprecating bscpylgtv.
• Multiple active TV profile selection.
• Removing installer Python dependency setup.