feat: import common BMP formats with lossiness reporting

[kbuffardi]

Summary

• replace native-struct BMP parsing with checked little-endian decoding and atomic failure behavior
• import CORE/INFO/V2/V3/V4/V5 BMPs across indexed, RGB, bitfield, RLE4, and RLE8 encodings
• add Bitmap::isLossy() with the confirmed alpha, color-precision, and color-metadata semantics
• preserve stored RGB values when alpha is discarded and keep 24-bpp BI_RGB save output
• document supported inputs, lossiness behavior, safety limits, and unsupported formats

Supported input

• BITMAPCOREHEADER: 1/4/8/24-bpp uncompressed
• BI_RGB: 1/4/8/16/24/32-bpp
• BI_BITFIELDS: 16/32-bpp, including declared alpha masks
• BI_RLE4 and BI_RLE8: encoded/absolute runs, EOL, EOB, delta, and alignment
• bottom-up images plus top-down uncompressed/bitfield images

Embedded JPEG/PNG, CMYK, later OS/2-specific variants, and color-profile transforms remain explicitly unsupported and are documented in the README.

Lossiness contract

isLossy() is true when import discards color information or precision:

• a declared alpha channel contains at least one non-opaque sample
• an RGB bitfield component wider than 8 bits is quantized
• meaningful V4/V5 color metadata is retained only as numeric RGB values without applying its transform

Palette/RLE expansion, lower-depth RGB expansion, row orientation, padding removal, opaque alpha, and BGRX reserved bytes remain lossless. Spatial dimensions are never resampled.

Safety

• validate signatures, known DIB layouts, dimensions, planes, compression/depth pairs, offsets, palette regions, masks, strides, profile ranges, and decoded-size arithmetic
• reject out-of-bounds RLE runs/deltas and truncated command streams
• decode into temporary state and commit only after full success
• cap input files at 512 MiB and decoded images at 100 million pixels
• add no decoder dependencies

Verification

• ./test_runner.sh
• g++ -std=c++11 -Wall -Wextra -pedantic -c bitmap.cpp -o /tmp/bitmap.o
• g++ -std=c++11 -Wall -Wextra -pedantic -fsanitize=address,undefined -fno-omit-frame-pointer bitmap.cpp tests/bitmap_tests.cpp -o /tmp/bitmap-tests-sanitized
• /tmp/bitmap-tests-sanitized
• g++ -std=c++11 -Wall -Wextra -pedantic example.cpp bitmap.cpp -o /tmp/bitmap-example (only the pre-existing untracked example emits signed/unsigned loop warnings)
• git diff --check master...HEAD

Closes #1