fix(protocols)!: keep pack() from rewriting the instance it is called on (#1769)

[JarryShaw]

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Summary

pack() assigned the header it built to the instance, so UDP(srcport=1234).pack() left schema.srcport at 0 while info.srcport kept 1234. 1.4.1.post2 did the same. Per the owner's choice (option A on #1769), pack() with no keywords now builds a fresh header and leaves a parsed or constructed instance unchanged.

• protocol.py: once the instance exists, _packing_target() gives pack() a shallow copy, so neither the header nor state make() sets (TCP's connection flags) reaches the instance. Construction still keeps its header for unpack().
• frame.py, pcapng.py, httpv2.py: their overrides had the same defect. PCAP-NG's copy also gets its own option counter, so re-packing a block with an only-one option no longer raises.
• Docs: pack() states the contract, with versionchanged:: 1.5.0.
• Breaking: code that relied on pack() re-syncing schema now has to call make() or reconstruct.
• Bite: with pcapkit/ reverted the new module fails 17 tests; reverting each override fails its own test.

Closes #1769

[JarryShaw]

Verdict: GOOD TO GO at 4cef30e54. A Sonnet cross-review (author: Opus) checked f4a0384ca; this head adds only ! to the subject, and the tree is identical. It matches the owner's option A. Reverting each fix fails its own test. An AST scan and a vars() snapshot over 30 protocols found no override the fix missed. The returned bytes are unchanged. 1.4.1.post2 mutated too, which justifies the versionchanged.