runtime: make the log pipeline pluggable through a ComponentBuilder seam

[mfw78]

Follow-up to the per-component builders (PR #190, commit dd9cbe7).

Problem

ComponentsBuilder::build bakes in the log backend:

let logs = LogPipeline::in_memory(ctx.config.limits.logs());

Chain, store, and the lattice Ext are each pluggable through a ComponentBuilder, but LogPipeline is not. An embedder who wants a custom log sink cannot substitute it without reconstructing Components by hand.

Proposed

Give the log pipeline the same seam: a LogPipelineBuilder (with a default reproducing todays in_memorybehaviour, analogous to the()no-opComponentBuilderfor a core-onlyExt), added as a fourth member of ComponentsBuilder<C, S, E, L>`.

Scope

Changes the ComponentsBuilder generic signature, so it ripples to the builder/launcher cars that construct it (#193/#194). Kept out of #190 for that reason; own PR, ideally alongside the add-ons/observability work.

[mfw78]

Moved to M2 (generic venue-agnostic host) and attached to the host-generalization epic #374. The pluggable log backend is part of the same ComponentBuilder and extension-seam rework (#375, #381), so it should land once with the seam generalization rather than being built in M0 and reworked in M2.

[mfw78]

Landed via #442 (squash-merged into dev/m1). Closing manually since the merge target is not the default branch.