<regex>: Remove dynamic array resizing from _Buf

[Julian Müller (muellerj3)]

After the most recent change, all _Buf objects are populated by a single _Insert2 call. This means that we can immediately allocate the correct amount of memory instead of dynamically growing the internal array.

This PR replaces the code for dynamically resizing _Buf's internal array by two new functions _Assign() that perform a single allocation of the correct size and initialize the allocated memory. As a result, the capacity member _Sz has lost its raison d'être (but we must retain it to preserve ABI).

As a char-like type [re.general/2], _Elem is required to be trivially copyable [string.general/2] (and <regex> doesn't support constexpr evaluation yet), so we do not have to explicitly construct and destruct the elements.

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[Copilot]

🟢 Approval recommended

All assignments use known non-empty inputs, preserve bounds checks, and maintain the existing ABI layout.

Pull request overview

Simplifies regex character buffers by allocating their final size in one operation.

Changes:

• Replaces incremental _Insert2 growth with _Assign overloads.
• Preserves _Sz for ABI compatibility.
• Updates all buffer-population call sites.

File summaries

File	Description
stl/inc/regex	Implements exact-size buffer allocation and updates callers.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Balanced (auto)

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