fix(irish): publish root_vowel_count, drop dead is_short_vowel helper

Two CodeRabbit findings that were still valid on the merge candidate:

- 02_stress: seg_root_vowel_count was computed but never written to context,
  so context.root_vowel_count (which the pass header advertises and downstream
  code reads) was always nil.
- 11_unstressed_reduction: a dead local is_short_vowel() referenced an
  undefined `ustring`; it would have raised if called. run() already inlines
  the equivalent check.

Ported from the benchmarked source repo, so the two copies stay byte-identical.
Verified: benchmark unchanged (C 96.92 / M 93.76 / U 94.84), golden 81 pass,
transcription e2e 13/13 pass.

Author: hellpanderrr

wiktionary_pron/lua_modules/ga-passes/02_stress.lua

@@ -449,6 +449,10 @@ return {
     end
 
     context.is_monosyllabic = seg_is_monosyllabic
+    -- Publish the root-syllable count alongside is_monosyllabic. The header
+    -- documents this pass as computing root_vowel_count, and downstream passes
+    -- read context.root_vowel_count; without this it was always nil.
+    context.root_vowel_count = seg_root_vowel_count
     return tokens
   end,
 }

wiktionary_pron/lua_modules/ga-passes/11_unstressed_reduction.lua

@@ -43,15 +43,10 @@ local AFTER_C_G_GUARD_EXCEPTIONS = {
 local SHORT_VOWELS = { ["a"] = true, ["e"] = true, ["i"] = true, ["o"] = true, ["u"] = true,
                        ["ɛ"] = true, ["ɪ"] = true, ["ɔ"] = true, ["ʊ"] = true }
 
--- Check if phon is a short vowel (no length mark)
-local function is_short_vowel(phon)
-  if not phon or phon == "" then return false end
-  -- Phon containing ː is long — never reduce
-  if phon:match(ustring and "[".. (ustring and ustring.len and "ː" or "ː") .."]") then
-    return false
-  end
-  return SHORT_VOWELS[phon]
-end
+-- NB: a local is_short_vowel() helper used to live here. It was dead code that
+-- referenced an undefined `ustring` (never required in this file), so calling it
+-- would have raised. run() inlines the equivalent test as
+-- `not phon:find("ː", 1, true) and SHORT_VOWELS[phon]`. Removed 2026-07-31.
 
 return {
   name = "unstressed_reduction",