Define canonical Workshop artifact formats and SourceMap

[e54-bot]

Parent decision: #271 · ADR-0013 (docs/adr/0013-source-mapping-across-provider-boundary.md)

Readiness: ready-for-implementation. First in the chain; the LPP, opy-rs, and Wright follow-ups depend on it.

Goal

Implement the canonical Workshop artifact formats and the public SourceMap defined by ADR-0013, and the shape guard on attached source mappings.

Scope

• Document workshop-rs/text-v1 and workshop-rs/mapped-text-v1 (file table, program shape, position-keyed spans).
• Public SourceMap: extract from a span-bearing Program; apply to a Program parsed from the same Workshop text; reject the whole mapping on shape mismatch.
• Shape guard: attached mappings record the shape they were attached to; span accessors return None after structural drift.
• Mapping granularity per ADR-0013: rule, condition, action, direct action argument, declarations.

Non-goals

• Nested value mappings (additive later).
• #[non_exhaustive] on Rule / Condition (tracked in Freeze the workshop-rs public API for 1.0 #252).
• Protocol negotiation or provider/consumer integration.

Acceptance criteria

• Both formats are documented in docs/source-preservation.md and the public API docs.
• SourceMap extract → emit → parse → apply round-trips every mapped position on real-project fixtures.
• A shape mismatch rejects the whole mapping with a typed error; no partial application.
• After inserting or removing a rule, condition, or action, affected span accessors return None (tested).
• Columns use Position units (1-based Unicode scalar values), tested with non-ASCII source.
• cargo-semver-checks shows the change is additive.