Replace DocLayout with validated LayoutPlan for total coverage

[ryo33]

Summary

This PR replaces the best-effort DocLayout projection system with a new LayoutPlan that provides total, verified coverage of all reachable document nodes. The new system eliminates silent fallbacks and makes all failure cases explicit through typed PlanError variants.

Key Changes

• Removed crates/eure-document/src/layout.rs: The old DocLayout, LayoutStyle, and project_with_layout are deleted entirely.

• Added crates/eure-document/src/plan/ module with three submodules:

  • plan.rs: Core LayoutPlan and PlanBuilder with explicit form assignment and validation
  • plan/emit.rs: Emission logic that converts a validated plan to SourceDocument
  • plan/traverse.rs: Reachable-node traversal helpers

• New seven-variant form taxonomy (Form enum):

  • Inline: path = value
  • BindingBlock: path { ... }
  • BindingValueBlock: path { = value ... }
  • Section: @ path with items
  • SectionBlock: @ path { ... }
  • SectionValueBlock: @ path { = value ... }
  • Flatten: Hoist children without self-emission

• Orthogonal array handling (ArrayForm enum):

  • Inline: Single binding path = [...]
  • PerElement(Form): Per-element with [] marker
  • PerElementIndexed(Form): Per-element with explicit [i] indices

• Explicit validation:

  • check_form_compat() and check_array_form_compat() validate form assignments against node kinds
  • dry_walk_validate() performs structural validation before emission
  • All errors are typed PlanError variants (no silent fallbacks)

• Updated schema integration (eure-schema):

  • materialize_layout_plan() replaces materialize_doc_layout() with Result<LayoutPlan, PlanError>
  • LayoutStrategy now aliases Form instead of LayoutStyle
  • Schema-derived strategies are applied with full error propagation

• Updated test suite and CLI:

  • Test cases now use LayoutPlan API
  • Schema files updated to use new form names (section-block instead of nested, etc.)
  • Error handling updated to surface PlanError in scenarios

Notable Implementation Details

• The PlanBuilder API is fluent: set_form(), set_array_form(), order() all return Result<&mut Self, PlanError> for chaining.
• Emission is split into validation (dry_walk_validate) and real emission (emit), both using the same control flow to ensure validation mirrors what emission produces.
• Extensions always precede value children in emission to keep tags like $variant at the top of blocks.
• The root node's form is implicit (cannot be set); only non-root nodes are assigned forms.

https://claude.ai/code/session_01TP4JqaAMFLYoMD5wzHESAs