fix: guard neighbour lookups in replace_blank (IndexError on trailing space)

[koriyoshi2041]

replace_blank (src/voxcpm/utils/text_normalize.py) crashes on text ending in a space, and mishandles text starting with a space.

for i, c in enumerate(text):
    if c == " ":
        if (text[i + 1].isascii() and text[i + 1] != " ") and (text[i - 1].isascii() and text[i - 1] != " "):
            out_str.append(c)
    else:
        out_str.append(c)

The intent is to keep a space only when it sits between two ASCII word characters, but the neighbour lookups aren't bounds-checked:

• A trailing space makes text[i + 1] raise IndexError: string index out of range. replace_blank("hello ") crashes. This is reachable from TextNormalizer.normalize() for any zh text that ends in a space.
• A leading space makes text[i - 1] read text[-1] (the last character), so the decision wraps around and a leading space can be wrongly kept.

Fix: only keep the space at interior positions (0 < i < len(text) - 1), so edge spaces are dropped (they aren't between two words) and no out-of-range access happens. Interior spacing (e.g. "a b" kept, "中 文" → "中文") is unchanged.

Added tests/test_text_normalize.py covering trailing/leading/interior spaces and a CJK-adjacent space.

Note: I verified the function's behaviour directly (it's pure string logic), but couldn't run the full test locally because importing the package pulls in torch/torchaudio and a native torchaudio lib fails to load on my machine — CI should exercise the added test normally.