dat: per-archive rebuilds, complete flags, hoist dat types to TU headers

[ribbanya]

• Per-archive type dependencies: samples types hashes the types each archive's roots reach and rewrites the hash only when it changes, so a header change rebuilds only the archives that use it (a stage type: the 71 Gr units, in 13 s).
• objdiff complete for units whose types explain the whole file: every relocation, every public symbol, no walk issues (49 of 765).
• Dat types declared in .c files are hoisted into their TU headers, so their data can be sampled; types unhoisted lists what's left (none). ftLoadCommonData, plLoadCommonData and the menu data tables are struct tags, since each shares its name with its archive symbol.
• Coverage: scene and stage model-group animation/light lists are DAT_NULLTERM, the stage model group's fields are named, and the debug and sound test name tables are sized (91.5% -> 95.0% of relocations explained).
• types duplicates finds records defined twice; ftData_x38 is merged into AbsorbDesc.
• Archive externs: named slots Melee's loader sets to NULL. The walk counts them as pointers, and the samples relocate them to their names.
• Generated headers group declarations as Samples, Externs and Elided; data that isn't written as C is a sized DatBlob array.

13564 of 13566 samples match, as before; the DOL still matches.