docs: add repository structure guide (no path renames)

[Davincc77]

Summary

Adds a single authoritative repository-layout document and links it from the schema index. This is a docs-only, additive change — no files were moved or renamed, and no package paths were touched. It is a conservative cleanup that makes the repo easier to navigate and makes future restructuring safe, without risking v4.1 stability.

• New docs/STRUCTURE.md documenting:
  • The top-level layout and the purpose of each directory.
  • Why schema/ (unified single-file schemas) and schemas/ (split envelope+payload schemas) are intentionally distinct and must not be consolidated — they are referenced by test vectors, release tooling, schema $id URLs, and ~40 doc links.
  • Why the root reference scripts (verify_vectors.py/.mjs, load_klickd.py, save_klickd.py) and historical snapshots (SPEC_v30.md, SKILL_v25.md, SKILL_v30.md, klickd_v330_spec.pdf) are kept at root (CI, CONTRIBUTING, issue templates, RFC links depend on them).
  • The preserved SDK package paths packages/@klickd/core and packages/pypi/klickd (never renamed/moved).
  • The x.klickd vs. internal codename boundary for public surfaces.
  • A Deferred / future migration section recording the moves that were intentionally not done and why.
• SCHEMA_INDEX.md: one-line pointer to the new structure guide.

Why no moves

Investigation showed that consolidating schema/<->schemas/, relocating root scripts, or moving root historical docs would each break load-bearing references (test vectors, .github/workflows/test-vectors.yml, release/bundle scripts, schema $id URLs, README/CONTRIBUTING/issue-template/RFC links). On a published v4.1 line (npm @klickd/core@4.1.0, PyPI klickd==4.1.0, Zenodo DOI 10.5281/zenodo.20459934) those are breaking changes, so they are documented as deferred rather than performed opportunistically.

Conflict avoidance

Touches only docs/STRUCTURE.md and SCHEMA_INDEX.md. Does not touch README.md, so there is no overlap with PR #98.

Testing

All run green after the change (docs-only, as expected):

• python3 verify_vectors.py -> 77/77 passed
• node verify_vectors.mjs -> 47/47 passed (13 skipped: hash-wasm not installed — expected)
• python3 scripts/validate_v4_schemas.py -> all v4 strict-schema validations passed
• python3 -m pytest tests/ -q -> 156 passed
• Verified every path referenced in docs/STRUCTURE.md exists.

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