Make peek value length configurable

[ousamabenyounes]

Fix #130

Summary

• add --peek-max-len=<len> with the existing 255-byte value limit as its default
• allocate one configurable peek buffer per trace context and use it for local, global, and PDO values
• document that long values also require a sufficiently large --buffer-size
• cover long local, global, PDO SQL, and PDO argument values plus allocation failure

Test proof

• RED before implementation: ./phpspy: unrecognized option '--peek-max-len'
• RED with the buffer-path fix reverted: glopeek_status=1 pdo_status=1; isolated PDO args: pdo_args_status=1
• GREEN full local suite on native amd64: Passed 17 out of 17 tests
• changed executable-line coverage: 23/23
• native amd64 shellcheck passed
• arm64 build and shellcheck passed under emulation; the hosted ubuntu-24.04-arm job provides the real arm64 runtime gate