Add centrallix-lib Utilities

[Lightning11wins]

This PR adds a more organized version of the utilities I created while working on #77, which have since been used in several other projects (which are now blocked by this PR).

[greptile-apps]

Greptile Summary

This PR adds four new utility modules to centrallix-lib — a diagnostic check macro set (check.h/check.c), a monotonic wall-clock timer (timer.h/timer.c), numerical range helpers (range.h/range.c), and byte/comma formatting functions in util.c — along with the necessary Makefile changes to include them in both the static and shared library builds.

• check.h: GNU statement-expression macros (check, check_neg, check_double, check_ptr) that capture and print diagnostics on failure; previously flagged issues (missing includes, multi-statement macro guards) are resolved.
• timer.c: Accumulating monotonic timer using NAN as a sentinel for the unstarted state; previous bugs (accumulation vs. assignment, clock_gettime unchecked return, NAN poisoning, NULL guard) are addressed.
• util.c: Adds snprint_bytes (human-readable byte sizes), snprint_commas_llu (comma-formatted integers), and fprint_mem (RSS memory report via /proc/self/statm).

Confidence Score: 5/5

Safe to merge; all changes are additive utility code with no modifications to existing logic paths.

The new modules are self-contained and only add code; no existing functionality is altered. The two findings are style-level issues (a missing do-while(0) on a debug-only macro, and duplicated cleanup in fprint_mem) that do not affect correctness.

centrallix-lib/src/util.c and centrallix-lib/include/timer.h are the two files with style findings worth a second look before merge.

Important Files Changed

Filename	Overview
centrallix-lib/src/util.c	Adds snprint_bytes, snprint_commas_llu, and fprint_mem utilities; logic is correct but fprint_mem duplicates fclose cleanup across two branches instead of using goto error as required by the team's style rule.
centrallix-lib/include/timer.h	Declares Timer struct and API; timer_benchmark macro uses bare braces instead of do-while(0), making it unsafe in unbraced if-else contexts.
centrallix-lib/src/timer.c	Timer implementation with correct start/stop/accumulation logic, null guards, clock_gettime error handling, and NAN sentinel for unstarted state; no issues found.
centrallix-lib/include/check.h	Diagnostic check macros with correct includes (errno.h, math.h, string.h) and GNU statement expressions for safe result capture; no new issues.
centrallix-lib/include/range.h	min/max/clamp macros guarded with #ifndef to prevent redefinition collisions; uses typeof for type-safe expansion.
centrallix-lib/src/check.c	Implements print_err_internal; correctly dispatches between perror, error-code, and generic stderr output paths.
centrallix-lib/src/range.c	Stub translation unit that only includes range.h (all logic is macro-based in the header); no issues.
centrallix-lib/include/util.h	Adds declarations for snprint_bytes, snprint_commas_llu, and fprint_mem with correct types; includes stdio.h for FILE*.
centrallix-lib/Makefile.in	Adds timer.o/lo, check.o/lo, range.o/lo to both XSTATICFILES and XDYNAMICFILES; correct and symmetric.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Caller] -->|timer_new| B[nmMalloc + timer_init]
    A -->|timer_start| C{isnan start?}
    C -->|yes| D[start = get_time]
    C -->|no| E[noop - already running]
    A -->|timer_stop| F{isnan start?}
    F -->|yes| G[noop - not running]
    F -->|no| H[total += get_time - start\nstart = NAN]
    A -->|timer_get| I{isnan start?}
    I -->|yes| J[return total]
    I -->|no| K[return get_time - start + total]
    A -->|timer_free| L[NULL guard → timer_de_init → nmFree]

    subgraph get_time
        M[clock_gettime CLOCK_MONOTONIC] -->|failure| N[return NAN]
        M -->|success| O[return tv_sec + tv_nsec / 1e9]
    end

    D --> get_time
    H --> get_time
    K --> get_time

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flowchart TD
    A[Caller] -->|timer_new| B[nmMalloc + timer_init]
    A -->|timer_start| C{isnan start?}
    C -->|yes| D[start = get_time]
    C -->|no| E[noop - already running]
    A -->|timer_stop| F{isnan start?}
    F -->|yes| G[noop - not running]
    F -->|no| H[total += get_time - start\nstart = NAN]
    A -->|timer_get| I{isnan start?}
    I -->|yes| J[return total]
    I -->|no| K[return get_time - start + total]
    A -->|timer_free| L[NULL guard → timer_de_init → nmFree]

    subgraph get_time
        M[clock_gettime CLOCK_MONOTONIC] -->|failure| N[return NAN]
        M -->|success| O[return tv_sec + tv_nsec / 1e9]
    end

    D --> get_time
    H --> get_time
    K --> get_time

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_{Reviews (5): Last reviewed commit: "Fix incorrect type format specifier." | Re-trigger Greptile}

[Lightning11wins]

This PR is ready for human review.