ECHO-8FFE-C246-E62C Fix backtracking protection. Adjusts pos/backtrack tracking inside the
path replacer so backslash and dot matches don't reset the backtrack
buffer prematurely, preventing ReDoS via crafted inputs.
Backported from https://github.com/pillarjs/path-to-regexp/commit/f01c26a013b1889f0c217c643964513acf17f6a4
(shipped upstream as 0.1.12). The index.js hunk is upstream verbatim;
the test.js hunk reuses upstream's test but is repositioned to land
inside v0.1.10's `describe('path-to-regexp', ...)` block (upstream's
diff context expected an intervening non-security commit not present
in 0.1.10).
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