feat: expose canonical action layout

[Teakowa]

Summary

• expose workshop_rs::emitter::action_width for canonical WIR action sequences
• derive layout from the emitter's existing recursive action expansion and central line path
• return explicit WIR/emission errors and cover leaf, nested If/Else, While, ForGlobalVariable, and ForPlayerVariable layouts
• index the public contract from the canonical docs/README.md

Scope

This PR does not add source-language control-flow semantics, Switch/Case/Default/DoWhile, labels, goto, CFG, fallthrough, break policy, or a relocation system. It does not modify opy-rs#61.

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo run --locked -p workshop-rs --bin workshop-catalog-gen -- check
• cargo test --workspace --all-targets --locked (249 passed, 24 suites)
• focused action-layout tests cover nested structured expansion, canonical-emission width consistency, and explicit invalid-WIR failure

Downstream contract

After this PR is merged and a release containing it is published, opy-rs#61 can replace its local normalized_action_width() model with workshop_rs::emitter::action_width(&program, &catalog, &locale, actions).map(|layout| layout.width), while retaining all OPY-specific switch/do-while semantics and diagnostics locally.

Closes #102

[Teakowa]

Review verdict: merge-ready

I reviewed the complete PR #103 diff against the narrowed scope and acceptance criteria of #102. I found no correctness, ownership, architecture, validation, or regression blocker.

Verified

• action_width() does not maintain a second manually synchronized width table. It invokes the same recursive Emitter::action() implementation used by canonical emission and counts at the shared line() sink.
• Nested If/Else, While, ForGlobalVariable, and ForPlayerVariable expansion is therefore derived from the same implementation that emits native Workshop structure.
• The API remains source-language-neutral. No Switch, Case, Default, DoWhile, label, goto, CFG, fallthrough, or source-language break semantics were added to WIR/workshop-rs.
• Invalid programs fail explicitly through InvalidWIR; emission/layout failures remain explicit through Emission rather than producing guessed/partial widths.
• The non-rule-final context is explicitly documented. This matches the relative-Skip use required by opy-rs#61 and preserves the existing rule-final If emission optimization instead of changing emitter semantics.
• Tests cover leaf/native width, nested structured expansion, emitter/layout agreement, and explicit invalid-WIR failure.
• The implementation is small and does not redesign WIR or introduce a generic target/relocation system.
• CI run #220 is green on the reviewed head, including Rust 1.85/stable quality gates, catalog checks, and the five real-project scenarios.

Minor / metadata

1. docs/action-layout.md is a durable public contract but is not linked from the canonical docs/README.md index. Add it under Architecture/ownership or a small compiler-consumer-contract section so the new public contract is discoverable.
2. The PR body is empty. The commit contains Fixes #102, but PR-level scope/closure should still explicitly link #102 (for example Closes #102) so the PR remains self-describing independent of merge strategy.

Neither item changes the implementation verdict. After those metadata/documentation cleanups, this is ready to merge and release; opy-rs#61 can then replace its local normalized_action_width() with the released workshop-rs API.