Clarify ledger artifact identity and roadmap priorities

[WhatsYourWhy]

Motivation

• Make artifact identity and path semantics explicit to avoid future tooling disagreements about what constitutes the "same artifact".
• Tighten guidance for RUN_FAIL traces to prevent nondeterministic data from leaking into semantic comparisons.
• Reflect that validator/diff tooling and schema discipline are first-class near-term priorities and ensure the manifest is explicitly derived from the ledger.

Description

• In docs/LEDGER_SCHEMA.md declare that artifact.logical_name defines semantic identity and that artifact.path must be a deterministic pure function of artifact identity, and add rules to normalize or omit stack traces for RUN_FAIL entries.
• In docs/ROADMAP.md update Near-term phases to: build deterministic runtime + ledger (Phase 0), deliver validator completeness and semantic diff tooling (Phase 1), and add branching and failure semantics (Phase 2).
• In docs/DESIGN.md add the line that the ledger is the source of truth and the manifest must never contain information not reconstructible from the ledger.

Testing

• None — these are documentation-only changes and no automated tests were run.

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Codex Task