docs: reconcile architecture guide with the fifteen-crate workspace

[shunichironomura]

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Review finding: M02 · Priority: P3

internals/codebase-reading-guide.md, section 2, says the workspace contains seven crates and lists only those seven, although the current workspace has fifteen. Later sections document some of the omitted crates, so the overview and detailed guide disagree.

Update the workspace inventory/dependency diagram when performing the planned refactors. Follow the project's rule to regenerate/review the suggested reading order when module dependency direction changes. There is no reason to modify the minimal README for this review.

Acceptance criteria

• Section 2 inventories all current workspace members from the root Cargo.toml (fifteen at review time).
• The overview and dependency diagram agree with the detailed crate sections.
• Review/update the suggested reading order if the dependency graph changes during related refactors.
• Leave the minimal README untouched unless an essential correction is independently required.

This documentation task can be implemented independently; reconcile the guide again after any module/dependency refactors.

Review context

Identified during the 2026-09-06 codebase review at workspace commit 7f2944b3d0d3d5a95cf1c10b7ad1ba7bb80228a4.