docs: describe the parser as vocabulary + positional layers in README

[derek73]

Reorganizes the README's how-it-works prose around the two-layer model of the parser:

• New "How It Works" section replacing the three loose paragraphs after the name-structure list: a vocabulary layer that recognizes pieces by what they are (the customizable sets — titles, suffixes, prefixes, conjunctions, nickname delimiters), and a positional layer that assigns everything unclaimed by where it sits.
• States explicitly that parsing is deterministic — no statistical models or machine learning; the same input always parses the same way.
• Names the load-bearing assumption directly: the positional layer assumes Western name order, replacing the vaguer "not likely to be useful for languages that do not conform".
• The "positional, not semantic" paragraph after the quick-start example is now a one-line callback to the positional layer (the "1 & 2, 3 4 5, Mr." example is unchanged).
• The Customization section now says why customization is mostly set-editing: it feeds the vocabulary layer.

All claims from the previous prose are preserved (conjunction chaining, prefix joining, capitalization correction, the pre/post-nominal aside, "not perfect, but it gets you pretty far").

Verified: docutils renders the README with no warnings (PyPI-safe) and the README doctests pass.

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.68%. Comparing base (c9e9f1b) to head (90b3c32).
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Additional details and impacted files

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  Coverage   97.68%   97.68%           
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  Files          13       13           
  Lines         906      906           
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  Hits          885      885           
  Misses         21       21           

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[derek73]

Closing — docs-only changes go directly on master rather than through PRs.