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>

tests/model_tests.rs

@@ -15,6 +15,15 @@ fn filter_default_accepts_everything() {
 
 #[test]
 fn mode_parses_from_str() {
-    assert_eq!("a".parse::<Mode>().unwrap(), Mode::A);
-    assert_eq!("c".parse::<Mode>().unwrap(), Mode::C);
+    assert_eq!("lexical".parse::<Mode>().unwrap(), Mode::Lexical);
+    assert_eq!("hybrid".parse::<Mode>().unwrap(), Mode::Hybrid);
+}
+
+#[test]
+fn mode_letter_aliases_still_parse_to_new_variants() {
+    // The legacy `c`/`a` CLI tokens remain accepted aliases (case-insensitive)
+    // for the renamed canonical `lexical`/`hybrid` modes.
+    assert_eq!("c".parse::<Mode>().unwrap(), Mode::Lexical);
+    assert_eq!("a".parse::<Mode>().unwrap(), Mode::Hybrid);
+    assert_eq!("A".parse::<Mode>().unwrap(), Mode::Hybrid);
 }