Add: high-level design doc with Mermaid diagrams

[0xLeif]

Summary

• Adds docs/HLD.md, a high-level design for SpecSync 6.0.0 with 10 Mermaid diagrams. It covers context, components, the check pipeline, the change lifecycle state machine, sequence diagrams for new/answer/approve, check --commit, review/finalize/ship and the CI + Trust PR path, an ER diagram of the change evidence files, and the release flow.
• Also covers the .specsync/ layout, the lifecycle digests and their domain tags, build and distribution, trust boundaries, failure modes, decisions, a glossary and a map of the existing docs. It links the existing docs rather than repeating them, and each claim cites the source file or function it comes from.
• Adds a short Architecture section to the README with one overview diagram and a link to the HLD. The rest of the README is unchanged.
• No SpecSync change package: docs/ and README.md are outside meaningful_paths in .specsync/sdd.json, and change audit --strict passes with 0 active changes.
• Pages: nothing changed. corvidlabs.github.io/spec-sync is a redirect shell to the CorvidLabs hub, plus the atlas badges. The HLD is not published there; GitHub renders the Mermaid in the repo directly.

Worth knowing while reviewing: section 4.4 of the HLD records that change check --commit and change ship --push stage with git add -A (git_commit_all in src/commands/change.rs). That stages untracked files anywhere in the tree. This PR only documents that behavior; it does not change it.

Test Plan

• All 10 HLD diagrams and the README diagram render with @mermaid-js/mermaid-cli 12.0.0
• Every relative link and in-page anchor in docs/HLD.md and README.md resolves
• fledge lanes run pre-push passes
• fledge lanes run verify passes (2498 + 437 tests, clippy, release build, strict spec check at 100% coverage)
• specsync change audit --strict passes (0 active changes)
• fledge trust verify --range origin/main...HEAD passes (Augur proceed, risk 27; provenance soft)
• hi check passes (159 criteria, 11 families)

🤖 Generated with Claude Code

https://claude.ai/code/session_01V3ZZAEiUP7xRJPozhZb6rL

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