refactor: split metadata.c into reader/writer

Separate read-only metadata accessors from write-path mutators to
prepare for the dual-license split (MIT reader / LGPL writer).

metadata.c retains 20 aaruf_get_* functions: geometry, media sequence,
creator, comments, media title, media/drive manufacturer/model/serial/
barcode/part number, firmware revision, CICM metadata, Aaru JSON
metadata, user/negative/overflow sectors, and image info.

metadata_write.c receives 28 functions: 15 aaruf_set_* (geometry,
media sequence, creator, comments, media title, media/drive
manufacturer/model/serial/barcode/part number, firmware revision,
Aaru JSON metadata) and 13 aaruf_clear_* counterparts.

No shared static helpers between the two files. No behavioral change.
All functions are independent public API — no function pointer or
callback mechanism needed (unlike close.c which required a finalization
hook).

Verified:
- Builds cleanly, zero warnings
- 163/165 tests pass (2 pre-existing failures)
- metadata.o exports no set_/clear_ symbols

CMakeLists.txt

@@ -256,6 +256,7 @@ add_library(aaruformat
             src/lisa_tag.c
             include/aaruformat/structs/lisa_tag.h
             src/metadata.c
+            src/metadata_write.c
             src/dump.c
             include/aaruformat/structs/tape.h
             src/blocks/tape.c