Add toTopic() channel operator for workflow-level topic assignment

Problem

Topics currently can only be written to via process output: blocks with topic: name. There's no way to forward an existing channel to a topic from a workflow context.

This forces a choice between:

1. Modifying upstream modules to add topic: outputs (breaks separation of concerns)
2. Creating relay processes that exist solely to forward data to topics (adds complexity/overhead)
3. Maintaining explicit emit chains through subworkflow hierarchies (extreme verbosity)

Use Case: nf-core/rnaseq

We're migrating nf-core/rnaseq to use workflow outputs (Nextflow 25.10+). The pipeline has ~130 outputs flowing through 3-4 levels of subworkflows. Each output requires:

Process emit → Subworkflow emit → Subworkflow emit → Workflow emit → Publish

This creates thousands of lines of channel wiring. We've even hit JVM string constant limits, forcing architectural changes purely to reduce code volume.

nf-core modules correctly emit outputs without knowledge of publishing—they shouldn't need to know about topics. But calling workflows should be able to route those outputs to topics trivially.

Proposed Solution

A channel operator to forward channel contents to a topic:

// Forward a channel to a topic
STAR_ALIGN.out.bam.toTopic('alignment_bams')
STAR_ALIGN.out.log_final.toTopic('star_logs')

// Later, in entry workflow publish block
publish:
alignment_bams = channel.topic('alignment_bams')
star_logs = channel.topic('star_logs')

Benefits

1. Separation of concerns: Modules emit normally, workflows control routing
2. Eliminates emit chains: No more passing outputs through every subworkflow layer
3. Natural aggregation: Multiple sources can forward to the same topic
4. Backwards compatible: Existing topic: in process outputs continues to work

Prior Discussion

This was requested in Discussion #2842 (2022) but never implemented.

Start by reviewing the existing process output topic behavior, channel operators, workflow-level output wiring, and the publish block described in the issue. Define how a channel-level toTopic() would interact with channel.topic(), multiple sources, subworkflows, and existing topic outputs; done means the behavior is specified, implemented, and covered by relevant tests.