runtime: make the log pipeline pluggable through the components seam

[mfw78]

Closes #265

What

Adds LogPipelineBuilder as a fourth ComponentBuilder slot on ComponentsBuilder<C, S, E, L = LogPipelineBuilder>, defaulting to today's in-memory backend. Runtime::components and the TypedBuilder/ComponentsStage/ReadyBuilder chain thread the new L generic. Adds ComponentsBuilder::with_logs to substitute a custom pipeline builder.

Why

Chain, store, and the lattice Ext are each pluggable through a ComponentBuilder; the log pipeline was hardcoded in ComponentsBuilder::build. This gives it the same seam so an embedder can substitute a custom log sink.

Testing

with_logs_substitutes_the_pipeline unit test asserts the bundle carries the exact pipeline yielded by a custom builder.

AI Assistance

Implemented with Claude Code.

Closes #265

[lgahdl]

The generic threading itself (TypedBuilder → ComponentsStage<...,L> → ReadyBuilder<...,L>, terminating cleanly at .build() before AssembledRuntime, which stays generic-free) is done correctly and completely — same shape as the supervisor_clock seam already threaded through this same chain. Default-generic call-site compatibility is fine too (ComponentsBuilder::new(c, s, e) still resolves L = LogPipelineBuilder). Two smaller things:

[lgahdl]

with_logs (like the chain/store/ext seams before it) is unreachable from any preset-based caller — PresetBuilder::launch calls R::components().build(...) directly with no hook to override the components builder first, so RuntimeBuilder::new(cfg).runtime::<CoreRuntime>().launch() can never reach it. This predates the PR (not a regression you introduced), but the PR body's "this gives it the same seam" framing slightly overstates parity, since none of the four seams are actually reachable via the preset shortcut today. Worth a one-line note in the description, or a follow-up giving PresetBuilder an escape hatch.

[mfw78]

Confirmed still valid at HEAD. PresetBuilder::launch (builder.rs:456) still calls self.preset.components().build(...) directly with no override hook, so all four seams stay unreachable via the preset shortcut (with_components at builder.rs:516 is on the non-preset TypedBuilder path only). The frozen PR-body framing is moot post-merge; the design gap is tracked in #509.

[lgahdl]

The trait declaration above spells out all four type params, but this impl's signature stayed at 3 (ComponentsBuilder<ProviderPoolBuilder, LocalStoreBuilder, ()>), silently relying on the L = LogPipelineBuilder default to satisfy the trait. It compiles today only because that default happens to match Self::LogsBuilder, but it reads as if this doesn't build a log pipeline at all — and the moment LogsBuilder changes to something else, this becomes a compile error at a line that looks unrelated.

Suggested change

	fn components() -> ComponentsBuilder<ProviderPoolBuilder, LocalStoreBuilder, ()> {
	fn components() -> ComponentsBuilder<ProviderPoolBuilder, LocalStoreBuilder, (), LogPipelineBuilder> {

[mfw78]

Confirmed still valid at HEAD. CoreRuntime::components (preset.rs:94) still returns the 3-param ComponentsBuilder<ProviderPoolBuilder, LocalStoreBuilder, ()>, riding the L = LogPipelineBuilder default; no later car spells out the fourth param. Tracked in #508.