feat: add automatic per-extension whitespace comparison - #729

Extend configuration to accept the existing booleans plus the opt-in string ignore_whitespace = "auto", keeping omitted/false as Normal and true as IgnoreAll, and add an ignore_whitespace_overrides TOML table mapping extensions to explicit booleans used only in auto mode. Supply a small documented built-in auto table (json, js, jsx, mjs, cjs, ts, tsx, rs ignore whitespace; unknown extensions, extensionless files, Python and YAML compare normally); user entries replace the decision for an extension, never invert their meaning, and describe this as a review heuristic rather than language-semantic equivalence. Issue #700 requests automatic whitespace comparison by file type, retaining whitespace changes for Python/YAML while allowing formatting noise to be ignored for JSON/JavaScript/Rust. Owner agavra explicitly welcomes a default configuration with user overrides; the reporter proposes extension-based configuration and warns that an exception list whose meaning reverses with another boolean would be confusing.

Config tests in src/config/mod.rs: omitted/true/false compatibility, auto defaults, explicit true and false overrides, empty table, inactive-table warning, invalid strings/types, malformed extension keys and normalized collisions. Existing invalid-boolean expectations must be updated deliberately for the expanded accepted syntax; Policy/selection tests in the new production module: uppercase extensions, multi-dot names, extensionless/dotfiles, non-UTF-8 paths where supported, destination-versus-source choice for renames/deletions, and reordered/missing ignored records. Verify original file order and that selected hunk coordinates/content hashes survive unchanged.

Fixes #700

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