Fix Zstd.decompress dropping all but the first of concatenated frames

[Watson1978]

Fixes #138

Problem

Zstd.decompress only decoded the first frame of concatenated zstd frames and silently dropped the rest. The zstd format explicitly supports frame concatenation (e.g. cat a.zst b.zst | zstd -d yields the concatenation of both), and ZSTD_decompress / ZSTD_decompressStream decode all frames, so the previous behavior diverged from the format.

a = Zstd.compress("Hello, ")
b = Zstd.compress("World!")
Zstd.decompress(a + b)
# => "Hello, "          (before)
# => "Hello, World!"    (after)

Cause

rb_decompress returned immediately after decoding the first data frame instead of looping back to process the remaining input. (The skippable-frame branch already advanced off and continued the loop; the data-frame branch did not.)

Fix

• decode_one_frame now reports how many input bytes it consumed (the final ZSTD_inBuffer.pos) via a new size_t* consumed out-parameter, so the caller can advance past the frame. The existing decompress_buffered caller is updated to pass NULL.
• rb_decompress scans the whole input, accumulating every frame's output:
  • A single DCtx is created once and reused across frames (decode_one_frame already resets the session via ZSTD_reset_session_only), and freed after the loop.
  • The first frame's output becomes the accumulator and subsequent frames are appended with rb_str_cat, so the common single-frame case stays zero-copy.
  • off is advanced by the consumed byte count and the loop continues; a consumed == 0 guard prevents an infinite loop.
  • Existing skippable-frame handling, magic scanning, the "not a zstd frame" rb_raise, and RB_GC_GUARD(input_value) are preserved.

Out of scope: dict:/kwargs handling, streaming, and skippable-frame behavior are unchanged. The public API signature and return type of Zstd.decompress are unchanged.

Verification

bundle exec rake spec passes (17 examples, 0 failures), with no new compiler warnings. New regression specs cover concatenated frames:

• a + b decompresses to "Hello, World!"
• a three-frame input (a + b + c) is fully concatenated

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