Implement the canonical public Workshop Program model

[Teakowa]

Parent: #112
Coordinates with: #32; the final 1.0 source-metadata attachment contract must remain compatible

Goal

Replace the current storage-shaped public WIR surface with the canonical public Workshop semantic model defined by #112 while retaining arena/ID-based storage internally where it remains useful.

Scope

• Make public Program, variables, subroutines, Rule, Condition, Action, Value, Event, and settings follow Workshop concepts rather than arena/node-ID storage.
• Keep raw Workshop actions as an ordered linear action stream: one Workshop action line maps to one public Action, including If, Else If, Else, While, For, and End.
• Keep Workshop values naturally composable as expressions and conditions explicit as rule-condition concepts.
• Make Workshop-native declaration/reference semantics sufficient for ordinary construction; external consumers must not manage arena allocation or WIR node IDs.
• Keep source/provenance optional for semantic construction and compatible with Define the canonical Workshop comment and source-preservation contract #32.
• Preserve private normalized storage only where it provides implementation value.

Non-goals

• A source-language lowering framework or policy.
• Typed builtin convenience generation; tracked separately under Define and migrate to the canonical Workshop-facing Rust API for 1.0 #112.
• Rewriting parser/emitter algorithms beyond what the new public contract requires.
• Unrelated storage or architecture cleanup.

Acceptance criteria

• A complete Workshop Program can be constructed from public Workshop concepts without touching Arena or storage IDs.
• Rule.actions exposes Workshop's linear execution structure rather than nested compiler control-flow nodes.
• Public semantic nodes are independently constructible and composable for whole-program or locally decomposed lowering.
• Generated programs require no synthetic source metadata.
• Existing raw Workshop semantics remain representable without a second public semantic model.
• Internal arena/ID representation, if retained, is not required knowledge for ordinary consumers.