fix: include raw remaining input as suffix in romaji_to_hiragana_predictively

[yukimemi]

Problem

When a query contains a kana-convertible prefix followed by an incomplete romaji
syllable (a bare consonant), mixed kana+ASCII filenames fail to match.

Concrete example (found in a Japanese launcher app using rustmigemo):

Filename	Query	Expected	Actual
インフラWBS	infura	match ✓	match ✓
インフラWBS	wbs	match ✓	match ✓
インフラWBS	infurawbs	match ✓	no match ✗

Root cause

romaji_to_hiragana_predictively("infurawbs") processes the string as:

1. infura → consumed into prefix as いんふら
2. w, b → pushed as-is (no romaji match)
3. s → triggers the predictive suffix block (terminal_nodes_for_prefix("s").len() > 1)

Inside the suffix block, only kana alternatives are inserted into the set
([さ, し, す, せ, そ, …]). The plain ASCII s is never added.

As a result, query_a_word_with_generator generates patterns like
インフラwb[サシスセソ…] but not インフラwbs. The case-insensitive regex
(?i)インフラwb[サシスセソ…] cannot match インフラWBS because S ≠ サ.

By contrast, querying "wbs" alone works because query_a_word_with_generator
adds the literal word via generator.add(&word_chars) at the top, so wbs is
already present as a pattern alternative. The problem only surfaces when the
ASCII tail is preceded by a kana-converted prefix.

Fix

One line added to romaji_to_hiragana_predictively — insert the raw remaining
input as an additional suffix option before returning:

// Also include the raw remaining input as a suffix option.
set.insert(query.to_vec());

With this change the suffix set for the trailing s becomes
[さ, し, す, …, s], so query_a_word_with_generator also generates
インフラwbs → (?i)インフラwbs matches インフラWBS. ✓

Side-effects analysis

The extra suffix is redundant in the no-prefix case: when the query starts
with an incomplete romaji syllable ("ky", "n", …) the raw input is already
added to the regex generator via generator.add(&word_chars) at the top of
query_a_word_with_generator, so the final regex pattern is unchanged.

In the mixed case ("infurawbs") the raw suffix produces genuinely new
alternatives (インフラwbs, いんふらwbs) that were previously missing.

Tests

• Three existing predictive tests updated to include the raw suffix in expected
  results (behavior change is intentional and correct).
• Two new regression tests added:
  • predictive_suffix_includes_raw_ascii_trailing_consonant — verifies "infurawbs" yields "s" in suffixes
  • predictive_suffix_includes_raw_for_single_trailing_consonant — verifies "s" alone yields "s" in suffixes

All 55 tests pass.