Specialize generated code to eliminate downstream Dialyzer warnings

[jallum]

Summary

• generate reader and writer clauses only for schema types reachable from the root
• precompute vector sizes and alignments instead of emitting runtime dispatchers
• preserve unsupported union-vector errors without generating unreachable member code
• add a downstream macro-consumer Dialyzer project to CI, with normal, safe, minimal, sparse, and union-vector-only schemas
• use no Dialyzer attributes, ignore files, or generated-code warning suppression

Evidence

The downstream fixture reproduced 21 Dialyzer errors and 10 skipped/unlexable warnings before the change. It now reports 0 errors, 0 skipped warnings, and 0 unnecessary skips with a forced PLT check.

• Elixir 1.18 / OTP 27: 142 tests, 0 failures
• Elixir 1.19 / OTP 28: 140 tests, 0 failures
• Elixir 1.20 / OTP 29: 140 tests, 0 failures
• flatc integration: 2 tests, 0 failures
• mix format --check-formatted
• mix deps.unlock --check-unused

Benchee on Elixir 1.19 / OTP 28 measured the generated decoder at 362K ips versus 143K interpreted, and the generated encoder at 94K ips versus 83K interpreted. Generated decoding retained its 19.77 KB allocation profile.

Tracks fb-3lk.