Use upstream APIs: drop build_meta, proper tail/text, zero-alloc navigation

Major refactor to use simdxml's python-bindings-api branch directly:

- Drop IndexWithMeta entirely — no more custom parents, name_ids,
  child_positions, or unique_names. self_cell dependent is now just
  XmlIndex directly.
- .text uses upstream direct_text_first() — zero allocation
- .tail uses upstream tail_text() — O(log n) binary search instead
  of O(n) substring search through raw XML
- .getparent() uses upstream parent() — direct array lookup
- .getnext()/.getprevious() use upstream child_position() + child_at()
- __len__ uses upstream child_count() — zero allocation
- __getitem__ uses upstream child_at() — zero allocation
- child_tags/descendant_tags use upstream child_slice() — zero alloc
- attrib/keys/items use upstream attributes() — single-pass parsing
- Tag interning built from upstream name_ids/name_table (no rebuild)
- Cargo.toml: simdxml dependency points to git branch (temporary,
  will switch to crates.io release)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cigrainger

Cargo.toml

@@ -9,5 +9,5 @@ crate-type = ["cdylib"]
 
 [dependencies]
 pyo3 = { version = "0.28", features = ["extension-module", "abi3-py39"] }
-simdxml = "0.1"
+simdxml = { git = "https://github.com/simdxml/simdxml", branch = "python-bindings-api" }
 self_cell = "1"