coff split: mark linker-written regions in Sections:, not as units

[handsomematt]

Follow-up to #13, from review on openblack/bw1-decomp#449: the region is already declared in Sections:, so mark it there instead of adding a unit for it.

Sections:
	.idata$5     type:rodata vaddr:0x008A9000 end:0x008A99C0 linker
	.rdata$debug type:rodata vaddr:0x008A99C0 end:0x008A99DC linker
	.idata       type:data align:4096 linker

What dtk does with a linker section or sub-region:

• carves it into its own autogenerated unit, named after the region (linker/idata_5, linker/rdata_debug, linker/edata);
• marks that unit linker as before: objs.rsp leaves it out, and the build config lists its ranges;
• doesn't split it on duplicate, comdat or alignment symbols, since the linker writes it whole.

Errors: the split fails if a splits.txt unit claims part of a linker region, or if generate_imports/generate_exports is on and an .idata/.edata split isn't in one.

Breaking change: the unit-level linker attribute from 0.0.36 is removed. Only the unmerged bw1-decomp#449 used it.

Verified on bw1-decomp BW1W120:

• 15/15 hashes match;
• every module's non-linker unit order is identical to 0.0.36;
• all 26 linker units are 100% complete.

cargo test passes and clippy reports nothing new.

🤖 Generated with Claude Code