fix(copy): PR #53 review round 3 — span gate terminology, mood tags, em-dash consistency

Fix "span gate threshold" → "span gate hum" (D-036 canonical term) in
detective opening, add missing mood tags on opening_m_d_002 and
the-terminal_d_020, normalize sera-venn double-dashes to em-dashes,
fix situations.yaml header count, document greeting fallback behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/design/layer2-situation-mapping.md

@@ -201,6 +201,8 @@ Content lines use `situation: [greeting]` as their tag. The engine currently pus
 
 **Schema note (dialogue-line.schema.json):** `"'greeting' is not yet in server/src/content/line_pool.rs — lines using it will be skipped until Rust is updated."`
 
+**Current fallback behavior:** Until the Rust update lands, lines tagged `situation: [greeting]` are excluded from the candidate pool entirely — they never fire. NPCs greet the player using their `situation: [routine]` or `situation: [social]` lines instead, filtered by access tier and mood as usual. This means Layer 2 relationship-aware greetings are functionally dormant; the content exists and is schema-valid, but the engine skips it. No error is raised — the line pool query simply finds no match for the `greeting` situation and falls through to other eligible lines.
+
 **Intended resolution (pending Rust update):** When `server/src/content/line_pool.rs` is updated to include `Situation::Greeting`, `greeting`-tagged content lines will fire. The Layer 2 relationship history distinction will then work through the access tier filtering that is already implemented:
 
 - `access: [public]` + `situation: [greeting]` → first meeting lines (Unknown relationship)