refactor: simplify hex escape pattern scanning

[nkuprins]

Related: #1000

Purpose of the pull request

The second of the two PRs. This one does the refactor/cleanup. No behaviour change.

What's changed?

• The match loop is written once instead of twice. The scan used to live in two methods: escapeHex located the
  first valid pattern, then processWithPatterns located every later one. Both carried the same match-handling code - bounds check, closing-_ test, hex validation, append, so any change to the rule had to be made in two places. It is now a single loop.
• The no-match path allocates nothing. The StringBuilder is created on the first match rather than up front, so a cell containing no _xHHHH_ - costs one indexOf scan and no allocation.
• The offsets have names. Each iteration derives hexStart, suffixIndex and patternEnd from PREFIX_LENGTH and HEX_DIGIT_COUNT.
• isValidHexFast is now isHexDigits, and the lookup table has a note on why it is not Character.digit(c, 16) - that method also accepts non-ASCII digits such as U+0663, which OOXML never encodes.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

[Copilot]

Pull request overview

Refactors EscapeHexCellWriteHandler’s _xHHHH_ pattern scanning to remove duplicated logic while preserving existing behavior, improving readability and avoiding allocations on the no-match path (as described in #1000’s refactor/cleanup step).

Changes:

• Consolidates pattern detection and replacement into a single scanning loop (removing the prior two-method split).
• Lazily allocates the StringBuilder only after the first valid match is found.
• Introduces named offsets/constants (PREFIX_LENGTH, HEX_DIGIT_COUNT) and renames hex validation helper to isHexDigits.

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[delei]

LGTM