Log qprintf Errors in xstring.c

[Lightning11wins]

Quick change that improves error handling of qprintf errors in xstring.c.

Blocked behind #97, which will obviously need to be merged before this PR is reviewed.

[greptile-apps]

Greptile Summary

This PR improves error reporting in xs_internal_QPrintf within xstring.c by creating a QPSession before each qpfPrintf_va_internal call so that per-error-type line numbers can be captured and logged via the new qpfLogErrors() helper. A large portion of the diff is a significant refactor of qprintf.c and an expansion of QPSession to store per-error line numbers in ErrorLines[QPF_ERR_COUNT].

• xstring.c: Switches from a bare printf to fprintf(stderr, ...) on failure and introduces a QPSession to call qpfLogErrors(), providing richer diagnostics.
• qprintf.h / qprintf.c: Adds ErrorLines[QPF_ERR_COUNT] to QPSession, rewrites the QPERR macro to record the triggering line, adds qpfLogErrors(), and fixes a %SSYB filter that was silently falling to NOTIMPL.
• Tests: Two new regression tests cover small-buffer quoting correctness and a buffer-underflow bug fix for %STR&QUOT.

Confidence Score: 5/5

Safe to merge; the session is always closed on all reachable paths and the fallback to NULL-session behavior is preserved when qpfOpenSession fails.

The xstring.c change is small and well-bounded: a session is opened after existing safety checks, used exactly once, and closed before the function returns. The large qprintf.c refactor is largely readability/doc improvement with one genuine fix (SSYB filter). No correctness issues found in the new error-tracking paths.

No files require special attention; the observations in qprintf.c are stylistic.

Important Files Changed

Filename	Overview
centrallix-lib/src/xstring.c	Core change: xs_internal_QPrintf now opens/closes a QPSession around each qpfPrintf_va_internal call for richer error logging; session cleanup is always reached, no leaks.
centrallix-lib/src/qprintf.c	Large refactor: adds ErrorLines tracking via updated QPERR macro, qpfLogErrors(), qpfSysMallocGrow, qpfPrintf_g/gva; fixes missing QPF_SPEC_T_SSYB in filter dispatch.
centrallix-lib/include/qprintf.h	QPSession gains ErrorLines[QPF_ERR_COUNT]; error constants rewritten as bit-shift literals; new public API declarations added.
centrallix-lib/tests/test_qprintf_69.c	New regression test verifying %STR&QUOT null-termination in a 2-byte buffer; assertions are correct.
centrallix-lib/tests/test_qprintf_70.c	New regression test verifying the buffer-underflow fix for %STR&QUOT with a 1-byte buffer; assertions are correct.
centrallix-sysdoc/QPrintf.md	Documentation updated to describe qpfLogErrors and the new grow-function APIs; copyright year updated.

Sequence Diagram

sequenceDiagram
    participant Caller
    participant xstring as xs_internal_QPrintf
    participant qprintf as qprintf.c

    Caller->>xstring: xsQPrintf / xsConcatQPrintf
    xstring->>qprintf: qpfOpenSession()
    qprintf-->>xstring: error_session (or NULL on OOM)
    xstring->>qprintf: qpfPrintf_va_internal(error_session, ...)
    note over qprintf: On error: QPERR sets Errors & ErrorLines[i]
    qprintf-->>xstring: rval
    alt "rval < 0"
        xstring->>qprintf: qpfLogErrors(error_session)
        qprintf-->>xstring: stderr output per error bit
    else "rval >= 0"
        xstring->>xstring: "this->Length += rval"
    end
    xstring->>qprintf: qpfCloseSession(error_session)
    xstring-->>Caller: rval

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_{Reviews (2): Last reviewed commit: "Fix invalid use of check_ptr()." | Re-trigger Greptile}

[Lightning11wins]

This PR is ready for human review.