docs: clarify the standards library starting points

[MrScripty]

Summary

• Explain the library's purpose and give readers a short path through Core Standards and the Router.
• Keep applicability authority explicit and connect tools to their existing setup and verification guides.
• Refresh the generated verification-input projection for the README digest and its added environment-guide target.

Current branch and scope

• PR #4, including the independently reviewed observability work and snapshot-revision fixture repair, merged as 0fc92a24279f3a4574c80af0a2f31ea347a21597.
• The README branch was updated without force to 4d6e755a886061e18582120e54e7f7e9bfe6fe26, retaining the previous README head and that new main commit as parents. The remote branch ref and commit were read back and verified.
• Against that main revision, the candidate changes exactly README.md and evaluation/standards-effectiveness/generated/suite-inputs.json. The previously reviewed prose is retained apart from removal of one extra EOF blank line. No unrelated tests, production source or validators changed.
• This PR remains a draft and is not merged.

Local verification

• Materialized the complete retained green source bundle, checked its published ZIP SHA-256 and confirmed its tree equals merged main.
• Used an isolated Python 3.12.14 environment installed from the unchanged requirements lock with --require-hashes --only-binary=:all:.
• Regenerated the companion through the supported Engine verify_repository operation with refresh_verification_inputs: true: 73 suites / 121 checks passed.
• Repeated a read-only checkpoint with refresh_verification_inputs: false: 73 suites / 121 checks passed.
• The formerly failing test_proposal_analysis_uses_one_projection_and_one_verified_base passed independently on the updated candidate (1 test, 3.462 seconds). Contract projection --check and git diff --check passed.
• Independent final-delta review verified all 13 relative README link targets exist, every pre-existing manifest record and top-level field is retained, and the only logical manifest changes are the README digest and its environment-guide link target. The merged fixture digest remains exact. No oracle weakening or accidental input loss was found.
• README SHA-256: 52aea11468d154104aa294717598f550dd1170e7ce144d5ff38a1752177a01a6.
• Generated companion SHA-256: e6cdf2bfa6a4eb0e9650a2ae6401c929be3e33240a9cb8de98c41087261c3039.
• The full serial Engine suite has not been rerun locally for this README candidate.

Hosted verification

• New-head hosted CI is pending. At the last read, GitHub's PR metadata still reported the preceding head despite the updated branch ref; no new-head workflow run or check had appeared. A full hosted pass is not claimed.
• Before this update, hosted CI on the old README head ran all 613 Engine tests and failed the unchanged semantic-revision fixture. The exact old main-base run failed the same assertion.
• Those historical runs stopped before complete repository verification. The separate repair's green hosted run passed all 636 Engine tests and complete verification (73/73 suites), on a tree verified identical to its merged main tree. That result does not substitute for this README candidate's pending hosted run.

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