Adopt PEP 822 dedentation and default t-string alignment

[grahamcracker1234]

Summary

Aligns dedent() with PEP 822 indentation and newline semantics while making multiline interpolation compose predictably across t-strings and f-strings.

• Closes Implement PEP 822 dedentation semantics #26
• Closes Enable align by default #10

Changes

• Implements exact space/tab prefix matching, closing-line indentation controls, and errors for incompatible whitespace-only lines.
• Aligns multiline t-string interpolations by default with per-value noalign, while supporting f-string alignment through align().
• Refactors interpolation rendering into a single-pass literal-and-hole pipeline and preserves runtime line endings.
• Splits tests by behavior, migrates snapshots to inline-snapshot, and updates development tooling.
• Rewrites the README with a focused hero example, version-specific usage, a concise textwrap.dedent comparison, and PEP 822 reference material.

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Note

High Risk
This is a breaking API and whitespace contract change across all dedent() callers, with a large refactor of core dedent/alignment logic and new error paths for malformed indentation.

Overview
dedent() now follows PEP 822 indentation: exact space/tab prefixes, closing-line control of trailing newlines, and IndentationError on incompatible whitespace-only lines. It drops the opening newline after triple quotes and removes the strip and align keyword arguments—other whitespace is left intact.

Interpolation is reworked so literal segments are dedented before holes are filled. t-strings align multiline values by default; {value:noalign} opts out. align() remains for f-strings and TypeError if used in t-strings.

The README is rewritten around the new behavior and rules. Tests move to inline-snapshot and split by concern; dev tooling (ruff, basedpyright, etc.) is bumped.

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[grahamcracker1234]

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