Replace dynamic builders with typed immutable state

[strider2038]

Replace reflection-based lann/builder state with private typed immutable state across SELECT, INSERT, UPDATE, DELETE, CASE, CTE, and statement builders. Persistent typed lists keep branching independent and avoid quadratic copying in long query chains. The root module now has no external dependencies; SQL rendering and final-pass placeholder numbering are retained.

Compatibility requires review before merging:

• Public method signatures remain unchanged, but conversions between concrete builders or to/from lann/builder.Builder, and external builder.Set/GetStruct calls, are no longer supported.
• Builder values remain comparable; equality represents state identity and no-op equality can differ.
• WHERE defaults on statement seeds now apply only to SELECT/UPDATE/DELETE. INSERT and CTE ignore these defaults instead of panicking on an unknown reflected field.

Adds a differential harness against published v0.1.0, concurrent branch/render regression tests, reproducible benchmarks, and CI checks. Representative construction-plus-rendering benchmarks use 53–65% fewer allocations (SELECT: 58 → 23); shared-host timing measurements improved in all measured scenarios but are not production performance guarantees.

Validation:

• Full scripts/test-all.sh passed on Go 1.26.2 with real PostgreSQL 18.1, including lint, unit/race, fuzz, differential comparison, dependency policy, module tidy, API inventory, compatibility, and integration checks.
• Go 1.25.14 and Go 1.26.2 additionally passed unit/race, vet, differential, compatibility, fuzz, and PostgreSQL 14.24 integration checks.
• Migration notes, changelog, API inventory, and benchmark evidence are updated.

See implementation report for raw benchmark links, compatibility details, and the remaining maintainer adoption decision. This PR is the separate post-v0.1.0 replacement stream; it does not publish a release.