NilPy: remaining pylib string builders still append per-byte (O(n²))

The dominant per-byte-concat sites (pyfile_slurp, pystr_upper, pystr_lower) were fixed (a50491d6) — amortised-doubling / preallocate instead of Result := Result + c every byte. The same O(n²) pattern remains in the smaller builders, confirmed as PXXStrConcat callers by callgrind:

• pystr_join (Result := Result + sep / + item) — total length is computable up front (sum of item lengths + sep*(n-1)); preallocate.
• PyReprQuote (repr with escapes) — grows a quoted string char by char.
• PyFmtBase / f-string formatting helpers.
• the strip family (pystr_strip/lstrip/rstrip[_chars]) — result length <= input; preallocate to input length and trim, or compute the span and Copy.

Each is O(n²) in the built string's length. None is on uforth's hot path (that was the file slurp), so impact is smaller — but the pattern is a latent cliff for any string-heavy NilPy program. Fix by preallocation where the final length is known, amortised-doubling where it is not (the pyfile_slurp shape).

Root option worth weighing: give managed AnsiString append a spare-capacity builder so s := s + c is amortised O(1) at the RTL, fixing every site at once — but that touches shared managed-string RTL (self-host gate), so per-site preallocation is the safer incremental path.

Partially fixed (this session) — pystr_join only

Rewrote pystr_join to preallocate the exact final length (sum of item lengths + sep repeated n-1 times) up front, then write in place — mirroring pystr_upper/pystr_lower's existing preallocate-and-index pattern. Each item is materialised into a local items array ONCE (VariantToStr isn't free to call twice per item) before the length sum, avoiding a second per-item conversion. Verified against CPython (including the empty-list and single-item edge cases) and the existing TypeError-on-a-non-str-item path, unchanged. Regression: test/test_nilpy_str_join_perf_fix.npy.

Still open, not attempted this pass: PyReprQuote, PyFmtBase/f-string helpers, and the strip family (pystr_strip/lstrip/rstrip[_chars]) — each its own, smaller preallocation fix in the same shape, left for a follow-up rather than done all at once under one ticket.