Speed up publicization by patching metadata in place

[krafs]

Publicization only flips visibility bits in the fixed-width Flags columns of the TypeDef, Field and Method tables, so the output can be produced by patching a copy of the input rather than having dnlib rebuild the entire metadata.

Measured on a 7.1 MB assembly, the write phase drops from ~1580 ms to ~5 ms, taking the whole cache-miss task from ~1.9 s to ~0.36 s. Cache hits are unaffected — they already skip the write.

Unpatchable layouts fall back to the existing dnlib writer: uncompressed/ENC metadata (#- heap, where deleted rows make row offsets unsafe to compute from rid), an unexpected Flags column name or width, or table extents past the end of the file. Each logs its reason.

Verified beyond the existing suites by diffing against dnlib's own output across three assemblies (7.1 MB / 5.1 MB / 1.1 MB): every member's flags match exactly, and Roslyn loads each patched image as a metadata reference with no diagnostics.

Two known gaps, tracked on the board rather than fixed here:

• The PE checksum is left stale (dnlib recomputes it). No loader in the test matrix cares, but it is a behavior difference.
• Strong-named inputs are unverified. The signature is already invalidated by today's dnlib rewrite, so this is most likely parity, but it has not been demonstrated.