chore(docs): document projection runtime boundary

[yordis]

• Keep the database-node boundary explicit before future feature work expands in-node projection responsibilities.
• Make projection runtime expansion external by default while preserving optional system projections for compatibility.
• Keep multi-stream append framed as a core write-semantics decision rather than incidental projection plumbing.

[cursor]

PR Summary

Low Risk
Documentation-only changes with no runtime, API, or configuration impact.

Overview
Adds docs/architecture.md and wires it into the docs site (sidebar.js, README.md) so contributors read the boundary before growing in-node features.

The new page states that the core node owns the durable event log (append/read, replication, storage, auth, etc.) and that user-defined projection runtimes, rich read models, and custom query surfaces are external components by default. System projections stay optional for compatibility but should not expand into a general in-node platform. Multi-stream append is framed as a core write-semantics decision, not projection plumbing.

projections.md gains a “Future direction” tip, clarifies system vs user-defined projection scope, and recommends external projection components for new read-model workloads when clients do not depend on built-in streams.

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[coderabbitai]

Walkthrough

This PR adds a new architecture.md document describing the boundary between core event-store node responsibilities and external projection/query components, links it from README and sidebar navigation, and updates projections.md with corresponding guidance.

Changes

Architecture direction documentation

Layer / File(s)	Summary
New architecture direction document docs/architecture.md	Defines core TrogonEventStore node responsibilities (event append/read, replication, storage, security) and establishes a boundary keeping compute/query/projection work external.
Cross-linking and navigation docs/README.md, docs/sidebar.js	README adds an "Architecture direction" section referencing architecture.md; sidebar adds architecture.md as a child page under "Server Quick Start".
Projections doc guidance updates docs/projections.md	Adds a "Future direction" callout and clarifications that system projections are compatibility features, discouraging expansion into an in-node projection runtime, with updated guidance on enabling system projections.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Poem

Hop, hop, through docs I go,
Drawing lines where boundaries show,
Core stays lean, event-log bright,
Projections roam outside the light,
A rabbit's map, now clear and right! 🐇📜

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the main documentation change about defining the projection runtime boundary.
Description check	✅ Passed	The description accurately summarizes the docs updates about external-by-default projections and multi-stream append semantics.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yordis/chore-document-projection-runtime-boundary

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@docs/architecture.md`:
- Around line 7-9: The architecture doc uses the internal name TrogonEventStore,
which is inconsistent with the public product name used elsewhere. Update the
wording in this section to use EventStoreDB consistently, and check nearby
documentation for any other occurrences of TrogonEventStore so the naming
matches the established docs.

In `@docs/projections.md`:
- Around line 38-43: The documentation text uses TrogonEventStore instead of the
established product name EventStoreDB, creating a naming inconsistency. Update
the prose in this projections section to use EventStoreDB consistently, matching
the terminology used across the rest of the docs and the referenced architecture
material.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 319bf0c0-2105-4f36-8256-4adfaa31782e

📥 Commits

Reviewing files that changed from the base of the PR and between 0f8c8a6 and 7a7c0f5.

📒 Files selected for processing (4)

• docs/README.md
• docs/architecture.md
• docs/projections.md
• docs/sidebar.js