feat(snippets): drop ld-application.slot

Per the schema audit, slot was a write-only field — declared on the
LDApplicationHints struct, decoded from frontmatter, never read by any
production code. The marker comment carries the snippet id directly,
so the per-page lookup slot was meant to power doesn't happen.

- Remove the LDApplicationHints struct and the LDApplication field on
  Frontmatter.
- Strip the `ld-application:\n  slot: <name>\n` block from all 122
  snippet files (KnownFields(true) would otherwise fail to parse them
  after the struct change).
- Render output unaffected: end-to-end against gonfalon shows
  `no changes` + `ok` from verify.

Author: kinyoklion

snippets/internal/model/model.go

@@ -24,9 +24,8 @@ type Frontmatter struct {
 	Lang        string           `yaml:"lang"`
 	File        string           `yaml:"file"`
 	Description string           `yaml:"description"`
-	Inputs        map[string]Input  `yaml:"inputs"`
-	LDApplication LDApplicationHints `yaml:"ld-application"`
-	Validation    Validation        `yaml:"validation"`
+	Inputs     map[string]Input `yaml:"inputs"`
+	Validation Validation       `yaml:"validation"`
 }
 
 type Input struct {
@@ -35,10 +34,6 @@ type Input struct {
 	RuntimeDefault string `yaml:"runtime-default"`
 }
 
-type LDApplicationHints struct {
-	Slot string `yaml:"slot"`
-}
-
 type Validation struct {
 	// Runtime selects the validator harness under
 	// validators/languages/<runtime>/. If empty, the snippet's `lang:`