Define sqlite3mcCodecAttach as SQLITE_PRIVATE to match its declaration

[LucaCappelletti94]

Description

sqlite3mcCodecAttach is declared SQLITE_PRIVATE but defined static. This defines it SQLITE_PRIVATE too.

Problem Statement

With -DSQLITE_PRIVATE="", which SQLite's testfixture build uses, the amalgamation fails to compile. Same error as in #4.

• Reproducible test case included
• Compiler / sanitizer evidence provided
• Verified incorrect behavior shown in code
• Reference to external technical source (if applicable)

Verification

$ gcc -fsyntax-only -DSQLITE_PRIVATE="" sqlite3mc_amalgamation.c
sqlite3mc_amalgamation.c:340419:1: error: static declaration of 'sqlite3mcCodecAttach' follows non-static declaration

Compiles cleanly with the fix. Default builds are unchanged.

• Locally reproducible
• Unit test added/updated
• Static analysis / sanitizer output
• Other (please specify)

Type of Change

• Bug Fix (requires verifiable evidence)
• Feature / Enhancement (new functionality)
• Refactor (no behavior change)
• Documentation
• Other (please describe)

Problem / Motivation

Blocks running SQLite's test suite against the SQLite3MC amalgamation.

Checklist

• I have independently verified the issue
• I am not submitting unverified or speculative changes
• I understand that AI-assisted changes must be reviewed by a human before submission