Add a bounds-checked binary cursor

[relh]

Summary

• add a small stateful cursor for sequential in-memory binary parsing
• expose catchable bounds errors, bounded slices and C strings, seeking, alignment, and subcursors
• provide explicit little- and big-endian integer and floating-point reads

This complements flatty/binny: fixed-offset reads remain in binny, while parsers that consume fields in sequence no longer need to duplicate cursor and bounds bookkeeping. The module is dependency-free beyond Flatty itself and contains no format-specific policy.

Validation

• nim check src/flatty/cursor.nim
• nim r tests/tests.nim
• git diff --check