Add APNG read/write support

[felixbuenemann]

Summary

• Add full APNG read and write support via new spng_decode_frame() and spng_encode_frame() APIs, supporting both one-shot and progressive modes
• Parse acTL, fcTL, and fdAT chunks with full validation (sequence numbers, frame bounds, dispose/blend ops)
• New public API: spng_get_actl(), spng_set_actl(), spng_get_frame_fctl(), spng_decode_frame(), spng_encode_frame()
• Upgrade libpng subproject from 1.6.37 to 1.6.55 with vendored APNG patch and fix for hIST chunk ordering bug
• Add 7 APNG test suites (basic roundtrip, sub-frame regions, error cases, progressive decode/encode, buffer vs stream, malformed input) — all 210 tests pass
• Update read and write fuzzers for APNG coverage
• Closes Animated PNG (APNG) read support #252

API

/* Decode frames from an APNG */
struct spng_actl actl;
spng_get_actl(ctx, &actl);  /* num_frames, num_plays */

struct spng_fctl fctl;
while (spng_decode_frame(ctx, buf, len, SPNG_FMT_RGBA8, 0, &fctl) == 0) {
    /* fctl.delay_num / fctl.delay_den = frame delay in seconds */
    display_frame(buf, fctl.width, fctl.height,
                  fctl.x_offset, fctl.y_offset,
                  fctl.delay_num, fctl.delay_den);
}

/* Encode APNG frames */
spng_set_actl(ctx, &(struct spng_actl){.num_frames = 3, .num_plays = 0});

struct spng_fctl fctl = {
    .width = w, .height = h,
    .delay_num = 100, .delay_den = 1000,  /* delay = 100/1000s = 100ms */
    .dispose_op = SPNG_DISPOSE_OP_NONE,
    .blend_op = SPNG_BLEND_OP_SOURCE
};
spng_encode_frame(ctx, img, len, SPNG_FMT_PNG, 0, &fctl);                    /* frame 0 → IDAT */
spng_encode_frame(ctx, img, len, SPNG_FMT_PNG, SPNG_ENCODE_FINALIZE, &fctl); /* last → fdAT + IEND */

Test plan

• All 210 existing tests pass (169 OK, 41 expected fail, 0 fail)
• 3-frame encode/decode roundtrip with pixel verification
• Sub-frame regions with offset verification
• Error handling (invalid acTL/fcTL, out-of-bounds, bad sequence numbers, duplicate chunks)
• Progressive decode and encode roundtrips
• Buffer vs stream output byte-identity
• Malformed APNG byte sequences (duplicate acTL, acTL after IDAT, bad sequence numbers)
• Read and write fuzzers updated for APNG paths

[felixbuenemann]

@randy408 please review