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/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.