fix(composite): preserve raw APEX ValueConv precision

[swackhamer]

Summary

• Split APEX ValueConv from PrintConv so Composite inputs receive raw seconds and f-numbers rather than formatted strings such as 1/152 or 4.7.
• Keep the existing EXIF display output unchanged by applying PrintConv only at CLI-formatting time.
• Add regression coverage proving the Composite path retains the raw 2^-7.25 shutter value.

Verification

• cargo fmt --all -- --check
• cargo test --lib composite::
• cargo test --lib exiftool_compat::