chore(claude-md): slim CLAUDE.md, move deep-dives into docs/

[jwalin-shah]

Summary

Extracts three long reference sections from `CLAUDE.md` into dedicated docs so they load on-demand instead of on every Claude Code session start. No content changes — same prose, new location.

Moved

• Testing guide (unit + E2E + mock backend + Rust tests) → `docs/TESTING.md`
• Event bus deep-dive (`DomainEvent` + `NativeRegistry`) → `docs/event-bus.md`
• Feature workflow + debug logging + controller migration + platform notes → `docs/feature-workflow.md`

Kept in CLAUDE.md

Still useful as always-loaded overview:

• Repo layout, commands, runtime scope, configuration
• Frontend quick map (provider chain, state, services, routes)
• Tauri shell overview
• Rust core key rules (controller-only exposure, light `mod.rs`, subdir layout) — brief form, pointer to full detail
• Coding philosophy, key patterns, git workflow

Size impact

• `CLAUDE.md`: 32.8k → 13.3k chars (~8.2k → ~3.3k tokens)
• Session context saving for contributors using Claude Code: ~5k tokens per session
• Contributors reading the repo in a normal editor lose nothing — pointers at the top link to the extracted docs.

Relation to other PRs

Complements #804 (`paths:` frontmatter for `.claude/rules/`) and issue #805 (rules audit). Together they target ~30-40k token savings per Claude Code session startup.

Test plan

• `docs/TESTING.md`, `docs/event-bus.md`, `docs/feature-workflow.md` render correctly on GitHub
• All cross-references in the new docs resolve (relative paths)
• Links from `CLAUDE.md` to the new docs resolve
• No content was dropped — search `git log -p main..HEAD -- CLAUDE.md` to confirm every removed paragraph landed in one of the three new docs

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⚙️ Run configuration

Configuration used: defaults

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Run ID: 92131ad8-1576-4062-a0c7-40370fb1d65f

📥 Commits

Reviewing files that changed from the base of the PR and between b3b451f and b19def6.

📒 Files selected for processing (4)

• CLAUDE.md
• docs/TESTING.md
• docs/event-bus.md
• docs/feature-workflow.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[jwalin-shah]

Superseded — main has independently absorbed similar slim/structured CLAUDE.md changes since this PR was opened. Closing rather than rebasing through CLAUDE.md conflicts. The downstream stack (#817, #818) has been rebased to drop this commit.