schema-lint v1 commit 2: DropMode + dormant Drop* variants

Second commit of the chassis v1 branch. Lands the type-level shape
of soft/hard drops without wiring them up. Variants are reachable
from emitters but the planner doesn't produce them yet; the apply
path returns an explicit not-yet-implemented error if one shows up
via deserialization.

Added:

- `DropMode { Soft, Hard }` — orthogonal to `SafetyTier`. Tier
  classifies the rule's risk class; mode is the operator's intent
  for data treatment.
    - `Soft` → catalog tombstone, data retained. Tier: safe.
    - `Hard` → Lance-level removal. Tier: destructive; will require
      --allow-data-loss to apply (commit 7).

- `SchemaMigrationStep::DropType { type_kind, name, mode }` and
  `SchemaMigrationStep::DropProperty { type_kind, type_name,
  property_name, mode }` variants.

- Re-export `DropMode` from `omnigraph_compiler::DropMode` so
  downstream crates don't reach into the catalog submodule.

- CLI `render_schema_plan_step` arms for both variants, surfacing
  the mode in plan output: `drop property 'Person.age' of node
  'Person' (soft mode)`.

- `apply_schema_with_lock` exhaustive match arm for the two new
  variants that returns `manifest_internal` with a clear
  not-yet-implemented message. If a SchemaIR JSON containing
  Drop{Type,Property} arrives (e.g. from a future tool or hand-
  written), the apply path fails explicitly rather than silently
  misclassifying.

- Two new in-source tests:
    - `drop_steps_round_trip_through_serde` — pins the wire shape
      for all four (variant × mode) combinations.
    - `drop_mode_serde_uses_snake_case` — pins external-tool-
      friendly serialization (`"soft"` / `"hard"`).

Build: clean, only pre-existing warnings.
Tests:
- omnigraph-compiler schema_plan: 6/6 (4 existing + 2 new).
- omnigraph-engine schema_apply: 11/11 (unchanged — planner still
  emits UnsupportedChange for removal paths).

Next commit (commit 3 per docs/schema-lint-v1-plan.md): add the
`tombstoned: bool` fields to NodeIR / EdgeIR / PropertyIR for the
catalog representation of soft-mode tombstones.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/omnigraph-cli/src/main.rs

@@ -1035,6 +1035,29 @@ fn render_schema_plan_step(step: &SchemaMigrationStep) -> String {
             type_name,
             render_annotations(annotations)
         ),
+        SchemaMigrationStep::DropType {
+            type_kind,
+            name,
+            mode,
+        } => format!(
+            "drop {} type '{}' ({} mode)",
+            schema_type_kind_label(*type_kind),
+            name,
+            drop_mode_label(*mode),
+        ),
+        SchemaMigrationStep::DropProperty {
+            type_kind,
+            type_name,
+            property_name,
+            mode,
+        } => format!(
+            "drop property '{}.{}' of {} '{}' ({} mode)",
+            type_name,
+            property_name,
+            schema_type_kind_label(*type_kind),
+            type_name,
+            drop_mode_label(*mode),
+        ),
         SchemaMigrationStep::UnsupportedChange {
             entity, reason, ..
         } => {
@@ -1073,6 +1096,13 @@ fn schema_lint_tier_label(tier: omnigraph_compiler::SafetyTier) -> &'static str
     }
 }
 
+fn drop_mode_label(mode: omnigraph_compiler::DropMode) -> &'static str {
+    match mode {
+        omnigraph_compiler::DropMode::Soft => "soft",
+        omnigraph_compiler::DropMode::Hard => "hard",
+    }
+}
+
 fn render_prop_type(prop_type: &omnigraph_compiler::PropType) -> String {
     let base = if let Some(values) = &prop_type.enum_values {
         format!("Enum({})", values.join("|"))