refactor: rename Mode variants C/A to Lexical/Hybrid (closes #2)

The retrieval-mode enum used opaque single letters `Mode { C, A }`;
neither the identifier nor the operator-facing `--mode a` token
conveyed what the mode does. Rename to self-describing names that map
directly onto the glossary definitions:

  Mode::C -> Mode::Lexical  (BM25 + tag filter, offline, no IONOS)
  Mode::A -> Mode::Hybrid   (lexical u semantic -> RRF -> rerank)

Decisions taken (the issue's open questions, resolved):
- CLI canonical tokens are now `lexical` / `hybrid`; the default is
  `lexical`. The legacy `c` / `a` tokens stay accepted as
  case-insensitive input aliases (zero cost; protects hand-typed
  invocations). A unit test pins the alias contract.
- The diagnostics `mode` string (Debug-derived) now emits
  "Lexical"/"Hybrid". Verified safe: no downstream consumer parses it.
  doctate is the future integrator and embeds the library (the `Mode`
  enum), not the JSON string (design spec §103).
- The reserved, out-of-scope generative tier (spec's "Mode B") keeps
  conceptual room: a future `Mode::Generative` does not collide with
  the retrieval-strategy names Lexical/Hybrid.

Surface: enum + FromStr, CLI defaults and `eval --mode both`
expansion, the private `collect_mode_c`/`collect_mode_a` methods (now
`collect_lexical`/`collect_hybrid`) and their comments, the two
pipeline mode test files (renamed), the eval CLI test, the glossary
(old letter forms demoted to Avoid lists), and the spec CLI synopsis.

closes #2

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/glossary.md

@@ -20,13 +20,13 @@ The BfArM Alpha-ID dataset mapping free-text diagnosis phrases to ICD-10-GM code
 **Avoid:** segment, unit, Einheit, Formulierung
 One caller-extracted diagnostic phrase (one input line) — the unit that is matched and ranked. In source code this concept is carried by the identifier `segment` (e.g. `split_segments`, `source_segments`), which is retained as a code term only.
 
-### Mode C
-**Avoid:** Modus C
-The lexical-baseline retrieval mode: BM25 over the corpus plus tag filtering, with no IONOS calls. Also the degrade target for Mode A.
+### Mode Lexical
+**Avoid:** Mode C, Modus C, mode c
+The lexical-baseline retrieval mode: BM25 over the corpus plus tag filtering, with no IONOS calls. Also the degrade target for Mode Hybrid.
 
-### Mode A
-**Avoid:** Modus A, semantic mode
-The hybrid retrieval mode: lexical and semantic (IONOS embedding) pools fused via RRF, then cross-encoder reranked. Degrades to Mode C on any IONOS failure.
+### Mode Hybrid
+**Avoid:** Mode A, Modus A, mode a, semantic mode
+The hybrid retrieval mode: lexical and semantic (IONOS embedding) pools fused via RRF, then cross-encoder reranked. Degrades to Mode Lexical on any IONOS failure.
 
 ### billable
 **Avoid:** abrechenbar, Abrechenbarkeit
@@ -42,11 +42,11 @@ The ClaML process of generating terminal sub-digit codes (e.g. `E11.90`) by appl
 
 ### RRF
 **Avoid:** —
-Reciprocal Rank Fusion: the rank-based method that merges the lexical and semantic candidate lists into one fused pool in Mode A.
+Reciprocal Rank Fusion: the rank-based method that merges the lexical and semantic candidate lists into one fused pool in Mode Hybrid.
 
 ### rerank
 **Avoid:** —
-The Mode A cross-encoder step that rescores the fused candidate pool against the query phrase, using the IONOS reranker model.
+The Mode Hybrid cross-encoder step that rescores the fused candidate pool against the query phrase, using the IONOS reranker model.
 
 ### corpus
 **Avoid:** Korpus, match corpus
@@ -58,11 +58,11 @@ The per-phrase, content-hash-keyed persistent store of corpus embeddings under `
 
 ### vector index
 **Avoid:** ANN index
-The in-RAM matrix of corpus embeddings searched by exact cosine similarity for the semantic pool. Built only when every corpus entry has a cached embedding; otherwise Mode A degrades.
+The in-RAM matrix of corpus embeddings searched by exact cosine similarity for the semantic pool. Built only when every corpus entry has a cached embedding; otherwise Mode Hybrid degrades.
 
 ### degrade
 **Avoid:** —
-Mode A falling back to Mode C when any IONOS step fails (embed, rerank, or a missing vector index); the request is still answered and `degraded` is set in diagnostics. Distinct from the 5-char-parent lookup fallback in `meta_lookup`.
+Mode Hybrid falling back to Mode Lexical when any IONOS step fails (embed, rerank, or a missing vector index); the request is still answered and `degraded` is set in diagnostics. Distinct from the 5-char-parent lookup fallback in `meta_lookup`.
 
 ### tag filter
 **Avoid:** —

docs/specs/2026-05-18-alpha-id-icd-suggestion-design.md

@@ -197,9 +197,9 @@ alpha-id index build [--sample N | --full] [--confirm]
                                   #   Korpus+ClaML parsen, bge-m3-Embeddings,
                                   #   Lexik-+Vektorindex persistieren (resumierbar)
 alpha-id suggest [FILE|-]         # Formulierungs-Liste (eine pro Zeile) → Top-10
-   --mode c|a  --top 10  --billable-only  --valid-only
+   --mode lexical|hybrid  --top 10  --billable-only  --valid-only
    --chapters I,J  --exclude-exotic  --json|--table
-alpha-id eval --mode c|a --cases N --seed S [--gold PATH]
+alpha-id eval --mode lexical|hybrid --cases N --seed S [--gold PATH]
 ```
 
 - Config-TOML: IONOS-Endpoint/Modellnamen/Pool-Größen/Pfad-Defaults, per Flags überschreibbar.