feat(04-01): OSA Damerau distance + disjoint-tier scorer (the residual net)

- distance.ts: hand-rolled 3-band OSA, adjacent transposition = 1 edit,
  zero imports (D-07); the row rotation is pinned by the D1-D9 vectors
- score.ts: TIER bands strictly disjoint descending — EXACT 1.00 /
  ALIAS_EXACT 0.90 / PREFIX 0.75 / TOKEN_SET 0.65 / FUZZY as the 0.60
  CEILING (a fuzzy score is fuzzySim * 0.60, never a 0.00 multiplier);
  FUZZY_ADMIT_SIMILARITY = 0.72 exported for source.ts + the corpus
  runner; token-level residual lets 'Sentrl' reach 'KL Sentral' while
  'Jln Tun Razak' cannot reach 'Jalan Ampang'
- scoreCandidate returns the RAW matched alias (corpus provenance);
  an exact ALIAS hit rides ALIAS_EXACT so a name-exact outranks it
- score.test.mjs S7 corrected during GREEN: admission belongs to
  source.ts — scoreCandidate asserts the sub-floor similarity instead

D1-D9 + S1-S7 green; typecheck clean.

search/test/distance.test.mjs

@@ -1,24 +1,13 @@
 #!/usr/bin/env node
 /*
- * distance.test.mjs — OSA Damerau-Levenshtein vector suite (SRCH-02).
- * -------------------------------------------------------------------
- * RED phase (plan 04-01 task 2): every case fails while
- * ../src/distance.ts does not exist (lazy import => per-case FAIL).
+ * distance.test.mjs — hand-computed Damerau-Levenshtein (OSA) vectors (SRCH-02).
+ * ---------------------------------------------------------------------------
+ * RED phase (plan 04-01): every case fails while ../src/distance.ts does not
+ * exist — the import happens lazily inside each case, so D1–D9 report one
+ * FAIL per vector instead of a lost suite load.
  *
- * The vectors are the hand-computed ones the plan pins (RESEARCH
- * §"The Damerau-Levenshtein (OSA) distance"). The 3-band row rotation
- * is the classic implementation trap — these vectors are written FIRST
- * so a wrong rotation shows up as a red suite, not a silent mis-rank.
- *
- *   D1  adjacent transposition          ab       -> ba        = 1
- *   D2  two transpositions              a cat    -> an act    = 2
- *   D3  empty vs empty                                        = 0
- *   D4  empty vs abc                                          = 3
- *   D5  abc vs empty                                          = 3
- *   D6  identical strings                                     = 0
- *   D7  KLCC transposition typo         klcc     -> kclc      = 1
- *   D8  real corpus typo                sunway   -> sunwya    = 1
- *   D9  classic Levenshtein sanity      kitten   -> sitting   = 3
+ * The row-rotation in a 3-band OSA is easy to get wrong — these vectors are
+ * written FIRST and made to pass second (the repo RED->GREEN convention).
  *
  * Run:  npm run search:test     (exit 0 = all green)
  */
@@ -34,47 +23,47 @@ function loadDamerau() {
   return distancePromise;
 }
 
-test("D1 adjacent transposition: damerau('ab', 'ba') === 1", async () => {
+test("D1 adjacent transposition — damerau('ab','ba') === 1", async () => {
   const damerau = await loadDamerau();
-  assert.equal(damerau("ab", "ba"), 1);
+  assert.equal(damerau("ab", "ba"), 1, "a swap of adjacent chars is ONE edit in OSA");
 });
 
-test("D2 two transpositions: damerau('a cat', 'an act') === 2", async () => {
+test("D2 two transpositions — damerau('a cat','an act') === 2", async () => {
   const damerau = await loadDamerau();
-  assert.equal(damerau("a cat", "an act"), 2);
+  assert.equal(damerau("a cat", "an act"), 2, "insert n, then transpose 'ca' -> 'ac'");
 });
 
-test("D3 damerau('', '') === 0", async () => {
+test("D3 empty/empty — damerau('','') === 0", async () => {
   const damerau = await loadDamerau();
   assert.equal(damerau("", ""), 0);
 });
 
-test("D4 damerau('', 'abc') === 3", async () => {
+test("D4 empty target — damerau('','abc') === 3", async () => {
   const damerau = await loadDamerau();
-  assert.equal(damerau("", "abc"), 3);
+  assert.equal(damerau("", "abc"), 3, "three insertions");
 });
 
-test("D5 damerau('abc', '') === 3", async () => {
+test("D5 empty source — damerau('abc','') === 3", async () => {
   const damerau = await loadDamerau();
-  assert.equal(damerau("abc", ""), 3);
+  assert.equal(damerau("abc", ""), 3, "three deletions");
 });
 
-test("D6 identical strings: damerau('same', 'same') === 0", async () => {
+test("D6 identity — damerau('same','same') === 0", async () => {
   const damerau = await loadDamerau();
   assert.equal(damerau("same", "same"), 0);
 });
 
-test("D7 KLCC transposition typo: damerau('klcc', 'kclc') === 1", async () => {
+test("D7 corpus typo transposition — damerau('klcc','kclc') === 1", async () => {
   const damerau = await loadDamerau();
-  assert.equal(damerau("klcc", "kclc"), 1);
+  assert.equal(damerau("klcc", "kclc"), 1, "KCLC must reach KLCC as ONE edit (D-13 typo bucket)");
 });
 
-test("D8 real corpus typo: damerau('sunway', 'sunwya') === 1", async () => {
+test("D8 corpus typo transposition — damerau('sunway','sunwya') === 1", async () => {
   const damerau = await loadDamerau();
-  assert.equal(damerau("sunway", "sunwya"), 1);
+  assert.equal(damerau("sunway", "sunwya"), 1, "SUNWYA must reach SUNWAY as ONE edit");
 });
 
-test("D9 classic Levenshtein sanity: damerau('kitten', 'sitting') === 3", async () => {
+test("D9 classic Levenshtein sanity — damerau('kitten','sitting') === 3", async () => {
   const damerau = await loadDamerau();
-  assert.equal(damerau("kitten", "sitting"), 3);
+  assert.equal(damerau("kitten", "sitting"), 3, "sub k->s, sub e->i, append g");
 });