Prefix public bitfield macros with LBT_ to avoid namespace pollution

[ViralBShah]

What

src/libblastrampoline.h is the public header, yet it leaked the unprefixed macros BF_CHUNK, BF_MASK, and BITFIELD_{GET,SET,CLEAR} into every consumer's preprocessor namespace. BF_MASK especially is a plausible collision.

This renames them to LBT_-prefixed names:

• LBT_BF_CHUNK, LBT_BF_MASK (internal helpers — renamed, no alias)
• LBT_BITFIELD_{GET,SET,CLEAR}

and updates the in-tree usages (config.c, libblastrampoline.c) plus the active_forwards docstring.

Backwards compatibility

The three macros documented for consumers (used to read the public active_forwards bitfield) keep unprefixed aliases:

#define BITFIELD_GET(array, idx)   LBT_BITFIELD_GET(array, idx)
#define BITFIELD_CLEAR(array, idx) LBT_BITFIELD_CLEAR(array, idx)
#define BITFIELD_SET(array, idx)   LBT_BITFIELD_SET(array, idx)

so existing code keeps compiling. Builds cleanly.

Possible follow-up (not done here)

The header also defines platform macros (_OS_LINUX_, etc.) that leak into consumers. Confining those is a larger, riskier change (they gate LBT_DLLEXPORT), so it's deliberately left out of this PR.

Draft for review.