feat(core): parse GP let-ring into a LetRing technique + tag (#75, part 1)

core/src/complement.rs

@@ -443,6 +443,7 @@ const fn span_label(technique: SpanTechnique) -> &'static str {
         SpanTechnique::HammerOn => "hammer_on",
         SpanTechnique::PullOff => "pull_off",
         SpanTechnique::Vibrato => "vibrato",
+        SpanTechnique::LetRing => "let_ring",
     }
 }
 

core/src/corpus.rs

@@ -36,6 +36,13 @@ use crate::structure::{ComplexityProfile, StructureMetrics};
 ///   `rights` key) keep loading and re-serialize losslessly. Rights status is
 ///   not derivable from content, so it is captured at curation time and cannot
 ///   be backfilled.
+///
+/// Tag taxonomy is intentionally *not* versioned here: [`SwancoreTag`] grows
+/// additively (e.g. `let_ring`, #75) and `SCHEMA_VERSION` tracks structural
+/// `ChunkMeta` changes (the optional-field, forward-compatible pattern above),
+/// not the tag set — a new tag only breaks readers that hard-reject unknown
+/// variants, a curation-tooling concern, not a corpus-structure one
+/// (decisions 2026-06-19).
 pub const SCHEMA_VERSION: u32 = 7;
 
 // ── identifiers ───────────────────────────────────────────────────────────────
@@ -106,6 +113,8 @@ pub enum SwancoreTag {
     Bend,
     Vibrato,
     PalmMute,
+    /// Let ring — notes left to sustain into following beats.
+    LetRing,
     NaturalHarmonic,
     ArtificialHarmonic,
     // ── rhythm ─────────────────────────────────────────────────────────────
@@ -144,6 +153,7 @@ impl SwancoreTag {
             Self::Bend,
             Self::Vibrato,
             Self::PalmMute,
+            Self::LetRing,
             Self::NaturalHarmonic,
             Self::ArtificialHarmonic,
             Self::Syncopated,

core/src/dump.rs

@@ -225,6 +225,7 @@ const fn span_name(technique: SpanTechnique) -> &'static str {
         SpanTechnique::HammerOn => "hammer_on",
         SpanTechnique::PullOff => "pull_off",
         SpanTechnique::Vibrato => "vibrato",
+        SpanTechnique::LetRing => "let_ring",
     }
 }
 

core/src/event.rs

@@ -147,6 +147,8 @@ pub enum SpanTechnique {
     PullOff,
     /// Vibrato.
     Vibrato,
+    /// Let ring — the note is left to sustain into following beats.
+    LetRing,
 }
 
 /// Where a technique came from — import-side provenance (ADR-0018; the

core/src/gp.rs

@@ -597,6 +597,9 @@ fn map_gp_note_marks(
     if effect.vibrato {
         push_span(SpanTechnique::Vibrato);
     }
+    if effect.let_ring {
+        push_span(SpanTechnique::LetRing);
+    }
 
     let mut marks = NoteMarks::empty();
     if effect.accentuated_note || effect.heavy_accentuated_note {
@@ -1090,6 +1093,21 @@ mod tests {
         assert_eq!(spans[0].evidence, TechniqueEvidence::explicit());
     }
 
+    #[test]
+    fn gp_let_ring_emits_explicit_span() {
+        // let-ring is a per-note sustain technique GP records (#75); it must
+        // surface as a LetRing TechniqueSpan so derivation/curation can see it.
+        let effect = GpNoteEffect {
+            let_ring: true,
+            ..GpNoteEffect::default()
+        };
+        let mut spans = Vec::new();
+        let _ = map_gp_note_marks(&effect, Ticks(0), Ticks(480), &mut spans);
+        assert_eq!(spans.len(), 1);
+        assert_eq!(spans[0].technique, SpanTechnique::LetRing);
+        assert_eq!(spans[0].evidence, TechniqueEvidence::explicit());
+    }
+
     #[test]
     fn dead_note_imports_as_dead_marked_note() {
         // A muted "X" note (NoteType::Dead) still carries a real (string, fret):

core/src/technique.rs

@@ -35,14 +35,15 @@ pub struct DerivedTechniques {
 }
 
 /// Spanning techniques in canonical (declaration) order.
-const SPANS: [SpanTechnique; 7] = [
+const SPANS: [SpanTechnique; 8] = [
     SpanTechnique::Slide,
     SpanTechnique::Bend,
     SpanTechnique::Legato,
     SpanTechnique::PalmMute,
     SpanTechnique::HammerOn,
     SpanTechnique::PullOff,
     SpanTechnique::Vibrato,
+    SpanTechnique::LetRing,
 ];
 
 /// `lower_snake_case` name for a spanning technique.
@@ -55,6 +56,7 @@ const fn span_name(t: SpanTechnique) -> &'static str {
         SpanTechnique::HammerOn => "hammer_on",
         SpanTechnique::PullOff => "pull_off",
         SpanTechnique::Vibrato => "vibrato",
+        SpanTechnique::LetRing => "let_ring",
     }
 }
 
@@ -68,6 +70,7 @@ const fn span_tag(t: SpanTechnique) -> Option<SwancoreTag> {
         SpanTechnique::HammerOn => Some(SwancoreTag::HammerOn),
         SpanTechnique::PullOff => Some(SwancoreTag::PullOff),
         SpanTechnique::Vibrato => Some(SwancoreTag::Vibrato),
+        SpanTechnique::LetRing => Some(SwancoreTag::LetRing),
         SpanTechnique::Legato => None,
     }
 }

core/tests/technique_tags.rs

@@ -91,6 +91,15 @@ fn maps_spans_and_marks_to_direct_tags_and_a_superset_name_list() {
     }
 }
 
+#[test]
+fn let_ring_span_derives_the_let_ring_tag_and_name() {
+    // let-ring is newly parsed from GP (#75): a LetRing span must auto-derive
+    // both the dedicated SwancoreTag::LetRing and the "let_ring" name.
+    let d = derive_techniques(&score_with(&[SpanTechnique::LetRing], NoteMarks::empty()), 0);
+    assert_eq!(d.tags, vec![SwancoreTag::LetRing]);
+    assert_eq!(d.names, vec!["let_ring".to_owned()]);
+}
+
 #[test]
 fn presence_only_so_repeats_dedupe_and_output_is_deterministic() {
     // The same technique across groups/notes is "present", counted once.

docs/decisions.log.md

@@ -1251,3 +1251,14 @@ Architectural decisions go to [`adr/`](adr/) instead.
   is duplicated and kept in step (both fronts now test the track-consistency
   rule). Accepted: `arrange` generation is untouched, and `web/dist` stays
   gitignored (CI rebuilds it on deploy).
+
+- 2026-06-19 — In the context of auto-deriving the `let_ring` tag (and the rest
+  of #75's tag taxonomy to come), facing Codex's point that a new serialized
+  `SwancoreTag` value is unreadable by older `SCHEMA_VERSION = 7` tooling (serde
+  rejects unknown enum variants), we decided for keeping the version at 7 and
+  growing the tag taxonomy additively, and against bumping per tag or once for
+  the whole #75 expansion, to keep `SCHEMA_VERSION` meaning what v1–v7 set it to
+  mean — structural `ChunkMeta` field additions under the forward-compatible
+  optional-field pattern, not a tag counter — accepting that a pinned pre-tag
+  build hard-rejects a chunk carrying a newer tag (a curation-tooling concern,
+  since griff's reader and writer ship together).